You're probably depressed. Sometimes it catches me off-guard and I don't even know I'm depressed until I'm so far down the pit that my wife has to yell down to me. Everything seems fine. I have nothing to feel that bad about and yet, I am still depressed. Let yourself cry a bit, but don't let yourself stay there. Watch a sad movie if you feel the need. Sometimes we need something like that for catharsis. However, watch a good comedy afterward! Let it be, then let it pass. It will, too.