i could give a snarky comment, but i'll hold it.
I really think it has something to do with the fact that it's a strong emotion, and like it or not that has physical effects.
Think of terminator 2, when Arnie asks the kid "Why do you cry?" it's a damn hard question to define.
As far as trying to get better, call up a buddy, go shoot some pool, watch a comedian you like... replace that sad emotion with a joyful one.