Trust me, I bet most of us wish we could! I know I do at least. We absolutely have the ability to open up completely (and love to do it), but it's a process that we only feel really comfortable with if there's an active participation by the other person. In other words, ask us questions! I feel presumptuous when I talk about myself; it's because of some weird notion that I don't want to force myself on people, so when you ask me a question about myself, I then feel like I've been given the green light to talk/boast about and reveal myself.
I think this socially awkward function serves as a driving factor in our careers, as I'm constantly trying to speak and express myself through music. For an INFJ therapist, it may be the desire to help and figure out himself by helping others to figure themselves out, or an INFJ politician might fight injustice in the system because he/she's fighting the hypocrisy from within him/herself. It's kinda like a mobius loop--the out channeling to the in, in an irrational, yet behold-able way.