I'm going to sort of pull ideas from what dissonance and CzeCze have said here and combine/play with them.
As an introvert, I consider the inner workings of my psyche, whether they be thoughts or feelings, my personal, mostly private, property. Going around with them all hanging out would be something like the equivalent of walking into a department store buck naked and maybe disemboweled. So when someone else is hanging out all over like that, I feel a kind of empathic embarrassment. I think I'm fairly comfortable with my emotions, but they are my emotions and not for public/general consumption the majority of the time.
Human beings are hardwired as social animals to have at least a minimal level of empathy and when someone goes around emoting all over the place, they impose on the emotional state of those around them. Some people are more okay with both sides of the equation than others. I, like heart, don't mind so much when it's someone close to me, because the relationship merits that level of disclosure and investment of emotional energy.
OTOH, I'm probably not going to get very close to someone who is constantly exploding emotions everywhere because it is too much of an intrusion on my own inner calm. In order to not have my own equilibrium upended, I have to mentally block that person and you can't have a very close relationship with someone you've empathically blocked.