Humans. We have great and wondrous wetware governing our actions and moods. But it gets buggy from time to time. Here's my own chart for dealing with the blues.
Other things like eating, drinking and complaining to anybody who will listen also work for a short while, but have significant adverse side effects.
Of all these, the first step—STOP—is the most important. There are no awards for going through life beating your head against the desk.
Inspired by Rands' The Builder's High.