It's emotions, especially in a workplace setting where you're expected to bottle up everything and present this cardboard, 2-dimensional version of yourself. Sometimes people crack under whatever mounting pressures they're facing and that's perfectly normal. I've seen a 60+-year old man, who was a kind, gentle 40-year company loyal stalwart, sobbing one evening because the new 30-something director was a complete jackass. It was one of the most heartbreaking things i've seen.
A couple years ago, i was having a nervous breakdown due to marriage and other stuff going on and was pretty deep into a depression. A coworker noticed it, because he went through it before and saw all the tell-tale signs. This is a macho, man's man type guy and when he pulled me aside in a conference room to talk it out with me. I eventually told him I contemplated suicide a few days ago and was down by the subway station at lunch just summing up the courage to jump. He had tears in his eyes, as did I.
That said, don't cry everyday and don't make a spectacle out of it.