I very much keep my well thought out & supported opinions to myself, which is why it takes a lot to coax them out. I don't have much interest in sharing them, because I dislike conflict (which could possibly result) & it takes a lot of energy to explain them.
There is also a MASSIVE difference between "can not" explain & "will not" explain. I did not say people should automatically appreciate my beliefs - they should appreciate the fact that I have chosen to open up to them because it takes a lot out of me to do so. No one should approach anyone with a sense of entitlement to their thoughts. A single "why?" response is likely to come off that way, especially considering the blank face & monotone voice some INTJs display. Not to mention, why would you ask "why" if you're disinterested because you've already decided their beliefs are unfounded? That attitude is a perfect example of why many INFPs don't share easily.
It seems to me this thread has the INTJs asking why INFPs clam up around them when discussing deeper beliefs/feelings; just because you don't LIKE the reason offered, does not make it untrue. If you won't adjust your style to accommodate the INFP, then don't expect one to open up to you & accommodate you. It works both ways.
Stringstheory explained it nicely as well.