Letting people get close to you. Oh, isn't it terribly risky? *shuddering*
I think INFJs have huge issues with trust. We have lots of layers, lots of defenses, and a public persona which can be quite misleading. (Many INFJ are mistaken for T, for example. Or, sometime, even for E. because they can be very outgoing while involved in something important to them) Now, the vulnerable depths we have are something we like to keep secret from most people, in order not to get hurt. We can trust people, but it takes time, patience, and not making mistakes. We will forgive people for a lot of things: we will never forgive a breach of trust.
Personally, with the chosen few I allowed myself to trust, I do put up with a lot: childishness, anger, lack of initiative, being judgmental…lots of things that really annoy me but if I love you than what the heck. AS LONG AS YOU ARE LOYAL.
Once this is gone, I am gone. Forever. There are no second chances. So of course we are careful before letting people get close to us. We would like to be spared the drama. We are probably still too thirsty for true sharing to be careful enough: I should say that most INFJs will tell you their mistake has been to trust too much, not too little.
And, as Aphrodite says, even when you think you know us, there are still layers you are not aware of. Even when we do trust, usually a small corner of us will stay strictly private. If just feels safer this way.