Here's a guess, based on personal psychology.
Other people's emotions have always seemed to me to be the scariest thing in the world. I spend my life trying to figure out how things work and how to predict things, but someone else's feelings can just jump in and interrupt my life at any random time.
The way I've always viewed my own emotions is this: they come up, I integrate them into my view of the current situation, then I try to come up with the most rational course of action taking my emotions and what I can see into account. When I do in fact bring up my negative emotions, tact is extremely important, because it minimizes the possibility of more bad things happening. But most of the time, I simulate an entire discussion about a negative emotion in my head and decide acting on it won't really bring me anything.
So when I actually do find it prudent to have a discussion about a negative feeling I have, it means I couldn't figure out a way to deal with it on my own. And it means I've tried really hard to. Because of this, the feelings I actually express are the most intense, most heavy ones I feel -- the ones I can barely handle.
I think SFPs may in fact be the types that are on average the MOST different from me in terms of expressing emotions. For them, it seems more like a constant flow, with many not being a huge deal. But I can't really deduce which ones are big deals -- there's a lot of data to work with. Assuming they're all not a big deal leads me to trouble (and is heavily negatively reinforced), so I devote a lot of energy to each emotional expression I see because it might happen to be the one that's a big deal.
After a while, the dynamic looks like this: I hold back 95% of my negative emotions, showing only the ones I really can't handle. Meanwhile, I'm processing tons of emotions from the other person, most of which, according to my own strategy, aren't even worth bringing up. So I begin to build resentment for having to process both people's emotions, and the other person not having the courtesy to hold back useless expressions of negative emotion. And the other person is probably annoyed at me for being distant and unexpressive. But I'm only distant because I'm devoting so much energy to managing this stuff. So I get even more defensive because I know why I'm distant and I feel like they think it's selfish, when I know it's the opposite.
So on the surface I probably look scared and helpless a lot of the time. And the emotions I have on the table are on average more intense than the other person's, because I already filter out the ones that aren't.
And I know how I look, and I hate it, which makes me thought-loop even more, etc.