It's the emotion center for the brain, especially negative emotions like fear and anxiety. Older parts are often "responsible" for keeping you alive, so breathing, heart beating, etc.

People don't like rejection. Evolutionarily rejection from peers/society means danger. Even if it's not dangerous now, the brain (especially the amygdala) reacts the same. People get anxious, defensive, etc. It takes time and reflection to accept that maybe what you said was wrong or socially unacceptable. Internet is too fast to reflect, it's much easier to respond in anger and then forget it all.