It is the same with people. INFJs very rarely spend a lot of time on anything that doesn't matter to them quite a bit in some way. They have a stronger need than many types to have the approval of those they are very close to. They want very badly for other people to understand them. The things they do and like are closely tied up with who they are at their very core. If there is rejection or disinterest in any of the things or people that they spend time with and which matter to them, it feels like a direct rejection of them. So, they will only share what you follow up on and show an interest in and they will also gauge your reaction before showing you more. Perhaps other types care less deeply about some of their opinions and pursuits (eg playing volleyball for fun or throwing out an opinion just to see how it sounds or stir up a debate) and so they are more able to detach themselves when others do not respond the way they might hope.
this description makes me want to share what is going on with me currently with an acquaintance/friend of mine who, i suspect, wants to be my good friend. she is an enfp and quite a bit younger than me. we are in the same career, so she sorta looks up to me in a lot of ways, which is kinda weird, but whatever. i immediately took a liking to her cuz we are both zany in the same ways and have a lot in common. it wasn't hard for me to be myself around her (myself that most people see, and maybe a little deeper). i don't have too much time on my hands for new friends, but a close friend of mine has sorta distanced herself cuz of some hard stuff she's going through, so there was a void in my life a little bit, in the friend department. new girl was fun and i enjoyed meeting and getting to know someone new. i felt like the makings for a super good friendship were there.
however. as we started hanging out more, some things started happening that were contrary to how i'd like a friendship to go. for one thing, she was always inviting other mutual friends around when we had a date to get together (the more, the merrier in her mind). so, i was at the point where i wanted to sorta go deeper with her, to get to know her better and to share more about myself, but i couldn't really because others were always around. strike 1. then when i could test the waters with sharing more deeply, she'd kinda listen/kinda not. she always just wanted to talk about our careers or some silly subject. it didn't take more than one or two attempts for me to quit trying. strike 2. after this happened, i realized the desire sort of left me for trying to get closer to her, because i felt like my attempts at sharing more about myself were met with insincerity. if i can't get closer, i really don't desire to try to get together much, cuz what's the point? i don't work with her. i don't live near her. and i wasn't able to get close enough to her when i tried to really want to pursue the friendship.
now she's still trying to be my friend. wants to be on the inner circle, like saying things about camping together and going to my new fav pub together, going to the lake together, and i agree with her when she says this stuff cuz i WANT to WANT to, but i realize down inside me she didn't really pass my tests, not that they are conscious tests, but when given the opportunity to discuss something important to me, she wasn't interested. finally, when she tries to call and i have my kids say i'm unavailable (because i am unavailable) she gets pissy when i call her back and says, "i'm unavailable right now," in a snotty tone mocking me (and this has happened several times). strike 3. if she can't understand my space issues, and respect them, there really is no way i can be really good friends with her.