Dave,

Chemicals, mood and behavior are all connected. Negative thoughts can lead to depression and that depression causes the seratonin in your brain to not circulate as much as usual. That can be corrected by taking an SSRI (Prozac, Paxil, others) but you could also work on changing the way you think about things and you could also do some vigorous physical exercise to produce some endorphins to lift your mood.

Ron