The most helpful thing to understand about INFP is that they are first and foremost deeply introverted. They are also incredibly sensitive. Some vast majority of their cognitive energy is spent simply feeling their feelings.
And they have so.many.feelings.
This is actually quite a lot of work on an energetic level, and can be exhausting for the INFP, particularly for INFPs who haven't developed skill or comfort expressing themselves, because then basically their whole life is happening inside their heads. The INFP is private and guarded naturally, so even the well-developed INFP is often reticent to express their deepest feelings. When they do, it would behoove their partner to lean in, get comfortable, and listen carefully. The INFP will clam right back up if they feel remotely judged, criticized, or unheard, and it will be a long while before they try again with you. (If they ever do).
It is not uncommon to find INFPs who habitually isolate themselves or push people away, often because they had negative relationship experiences in the past or even just because they fear negative experiences. (Remember they are extremely sensitive, so deep emotional wounds can take a long time to heal). A wounded INFP will protect that rich internal landscape with the tenacity of a pack of junkyard dogs. Admittedly, these self-protective INFPs are hard to build relationships with, but it is not impossible with patience, time, and practical affection.
And I will add here that it is entirely worth it, because any INFP is a wonderful partner when you really snag one. They will give you all the space and understanding you need, a quality that is especially valuable if you're a big, weird extravert who struggles to find a partner who accepts you fully. An INFP will embrace your idiosyncrasies willingly; in fact, they tend to prefer "weird" or unconventional people. And once they trust you, they actually like it when you encourage them to get outside of themselves. They can be hilarious, silly, fun, playful partners. They can be wonderful performers and storytellers, in part due to their rich imaginations. They are also great with kids, in part because they are somewhat childlike and innocent themselves. They do well with bright, optimistic, cheerful, peaceful, steady, independent partners who are undaunted by their dark spells.
Because INFPs (even under the best of circumstances) are prone to dark spells, depression, and self-pity. They are very hard on themselves and prone to feeling guilty or ashamed and getting stuck in cycles of feeling this way. And they will usually withdraw when they are hurt or overwhelmed. It's just how they are. So a partner of an INFP has to learn to curb the urge to draw them out forcefully, because it won't work. They will actually be most likely to dig their heels in and retreat further if you try to force them to open up when they don't want to or aren't ready. That said, they do need your encouragement to come back from the dark side, especially if you have done or said something to contribute to their shutdown. You will need to gently inquire as to what's going on with them, and what you can do or say to make it better, and wait patiently until they are ready to talk to you about it.
I know that sounds confusing.
Here's another way to say it: a small amount of withdrawal isn't a thing to fear from your INFP partner; in fact, it might be necessary for their well-being. This is mostly because there is so much going on inside of them all the time and so few outlets for it that sometimes they need to just check out. Sometimes they need you to just leave them alone for a while, and let them work through whatever they're working through. But there is also a point at which you may need to go in after them and drag them back out to the light. Again, if you have done something (even unintentionally) to hurt them, you will need to make it right somehow in an authentic way before they will be able to come back to the relationship fully. It is worth reiterating that they are extremely sensitive to criticism, and they take everything personally, so you may not even know you've done something harmful until they tell you (IF they ever tell you). The worst thing you can say at this point (and I've definitely learned this one the hard way myself) is a dismissive or mocking "How could that possibly have hurt your feelings?" which suggests they are hyper-sensitive and being a baby. Even if they are, they don't want to hear it, and it won't go well. So, a better way to say it is: "Okay, I'm sorry that what I said/did hurt your feelings. Thanks for telling me. What could I have said/done differently?" or "I don't ever want to hurt your feelings, so please tell me what I can do to make this right." They appreciate a genuine effort to understand them and their feelings, so this is always your best bet.
It is somewhat ironic that INFP is so likely to withdraw, because it costs them the connection they need to keep them steady. They have a contradictory combination of desires: equally desiring connection and safety and freedom. Please remember that no matter how much they love you or feel connected to you, you simply cannot fence in an INFP without encountering a great deal of resistance. The balance is in being there for them and showing care, but also permitting them space to roam and be the lone wolf they often feel they are. Don't take this to mean they don't care about you or that they don't even need you to some extent; they do want to feel like you are there, you are present, you are reliable. They need to trust you to be there when they come back. The more times you are still there when they return, the greater the trust can grow. The fact that you will end up with an utterly devoted INFP is worth all of this effort. (I promise.)
It can be frustrating for the partner of an INFP to know that there is a rich inner atmosphere, but that most of what is going on inside the INFP either can't or won't be translated. It can be doubly frustrating if you often sense that your partner is daydreaming when you'd like them to be present with you. You may have to remind them fairly often to come back to Earth. They might have been thinking nice things about you, but you wouldn't know it, because all you know is they aren't talking or opening up.
So, if you are a person who needs a great deal of positive reinforcement, you will need to ask for it, and your INFP will have to make a conscious effort to meet that need. It can be interpreted by a sensitive partner (especially an NF partner who measures the quality of the relationship in this way) as deeply selfish (even though INFPs usually do not see themselves as selfish or self-centered and would rather die than harm you) that they often fail to provide the affirmation their partner may need. Especially ironic given that INFP needs so much positive reinforcement themselves. It is partly that the INFP is so internal that they forget they have to actually say words, and it is partly that the INFP just thinks you should know how they feel.
It is also simply that INFP can get rather awkward trying to explain how they feel. If your INFP has a hard time telling you how they feel, but you really want to know, it can be helpful to ask them to write things down. They can be reticent to say words out loud, but write beautiful love letters or poems full of all the emotions you wish they would or could just show you in real life.
(Side note: INFPs are excellent songwriters, artists, musicians, etc in part because of all these feelings; it is important that somehow or another they find a positive outlet for all their feelings. Generally, they love and are deeply moved by music and words. Like, sometimes to tears. One way to connect with your INFP is to share songs and books with them that you love, or that you think they will love, and then talk about it.)
INFPs are very slow to make decisions, and can benefit from a partner who is more decisive than they are. They actually tend to appreciate bossy or Type A people. Some of this is that they really don't care about a lot of things and are pretty easy to please (like that classic "What do you want for dinner?" conversation. They will usually say, "I don't care, you pick", and they do actually mean that. They actually do not give any shits what you have for dinner, and will probably be happy with anything you choose.)
Part of it is also that they have a hard time making decisions, most often because they have conflicting desires and will take all day weighing the options. It's much easier sometimes to just let someone else decide things. That is the neutral impact of this "slowness to decide" thing.
The negative side of this can be that they'll stay at jobs or in relationships far longer than is reasonable, even when everybody around them knows they need to make a move, because they are so terrified of making the wrong decision. They worry that they will screw everything up. And they hate hurting people. (Not that anybody "likes" it, but INFP will do almost anything to avoid hurting anyone's feelings, even to their own detriment). They are perfectionists and they feel terribly guilty if they feel they have caused someone pain or harm. So, they carefully (too carefully, one might say) consider all the possible options and outcomes before eventually (forever) coming to a conclusion.
Hmm. What else? Other positives, lest you be scared off: INFP is very accepting of quirks. One of the most accepting of all the personality types. They generally don't mind accommodating a partner who has strong opinions or strange tastes. They won't bat an eye at your needing to have ice cream right this minute, even if it's zero degrees outside or the middle of the night. They won't laugh at your awkward dance moves, or criticize your cooking, or make fun of your OCD tendencies, or argue with you when you want to reorganize your sock drawer for the third time this week. You can be obsessed with any weird stuff you want to. They accept that you enjoy ____ activity that is the nerdiest thing on Earth. You be you, kid. Generally, an INFP partner/friend will completely accept your strange cravings, desires, conversational topics, interests, weird outfits, kinky sexual proclivities, everything.
Almost invariably they'll be like, "Okay, that's fine. Whatever makes you happy." Live and let live, that's how the INFP rolls.
…UNLESS/UNTIL you violate one of their deeply held values. Then, look out: they will get argumentative and stubborn and totally irrational and they will fight to the death. And forget logic; logic has nothing to do with this argument. Because principles. Because you are wrong and here's why.
(Later they might see how they were a little extreme. Maybe. You'll learn the things that aren't worth arguing about, and avoid those topics.)
But generally, if you give your INFP space and understanding and room to be their own weird introverted self, INFPs in relationships are peaceful, amicable, and easy to get along with. Generally, they don't ask for much. They don't criticize unless it seems necessary. They are low-maintenance. They will love you the most, for loving them, seeing them, appreciating them, and letting them just be themselves.
Update: Here is some advice for the INFP him/herself:
Update: Here is some advice for the INFP him/herself: