It's been a long process for me to learn how to relate to NFPs without feeling like it's going to blow up in my face, though some NFPs are easier for me to find rapport with than others. Granted, over time, I've acknowledged that this is something I need to address, not the entire NFP population as a whole Anyways, I have noticed that the more expectations the relationship has, the more tension may exist. For instance, my ENFP sister kind of expects a lot out of me as her sister, and can be hurt pretty easily if I violate those expectations in a major way (from her perspective at least -- which is what makes it so hard for me. We value very different things). But I've had NFP friends in the past that I've been able to relate strongly with, and don't always worry I'm about upsetting them in some way. But when it comes down to it, I've accepted Ti can be rough around the edges, and hard for Fi to understand. It goes both ways, but Ti may come across as hurtful and Fi is really only just confusing to me. It's really just helped me work on my Fe (and Ni, when it comes to NFPs), so I feel pretty grateful to have all of those people in my life. It's never my intent to come across as cold or uncaring.
So depending on the person/situation/current relationship/mood, I may steer clear of the "mysterious black box" or it may turn into a treasure. It varies. I think it may be the same for them too. An example that involves my sister again: we had a falling out this previous year, and have been growing close again these past few months. But we've kept a lot of the conversations light, and avoid anything related to that falling out, or anything too involved. However, religion is really important to her and important for her to discuss and share with others. She's expressed she doesn't know how to relate or talk to me on the subject, and I can tell she feels a little disappointed I don't engage. Unfortunately, I'm not too keen on getting into something so personal with an NFP that I'm on thin ice with. To protect myself, and to protect our relationship.
I've always sensed they want to be close to me and understand what is (essentially) Ti, but they don't know how, and are afraid of getting their feelings hurt. At other times, they really appreciate the insight. A lot of them really like my Ti + Ni and try to tap into that often, though I feel more appreciated when they admire my Ti + Se.