I agree it's also just a human thing. But so are MDD, BP and BPD, just more extreme in some respects. It's interesting that what happens to be more extreme are emotions (and empathy), which is what we're discussing here.

But of course the negative emotion(s) are interesting as well. Strong, potentially destructive shortwaves. And the anxiety disorders are interesting not because anxiety is something alien to humans, but because in those disorders it is (what's in a name?) more pronounced.

Edit:
MDD is interesting because it's not associated with strong (positive) emotions, but it's still like BP and BPD in these group dynamics. It's interesting that BP is like BPD because BPD is far less stable and more easily influenced. Nevertheless, the dynamics are pretty much the same while just one "normal" person is more likely to be a negative influence (that may reinforce stronger emotions but not due to resonance, but due to overall pressure).