Depression. It fluctuates like a fish out of water. The flapping can hit other people nearby. They can catch whatever you feel, and as a result, it can magnify. The feeling becomes worse, and spreads even more easily and quickly. Be careful when you are emotional. It is contagious. People are fragile animals.

Someone voted this down, most likely an emotional person offended by my cold response. Hey, I'm human too! As Blake put it:

Can I see a falling tear,
And not feel my sorrow's share?
Can a father see his child
Weep, nor be with sorrow fill'd?
(from "On Another's Sorrow")