Gee... good question. I myself suffer the ame thing and have been wondering for a few weeks now if I'm starting to slip into a depression.

This has already been said, but it bears repeating:
One thing that does help me to to actually FORCE myself to do something I used to do, but haven't in a while. So say, I used to love to do needlework (true I did) but I haven't done anything in over 4 years. I am considering whipping out my supplies and making something. I usually get happy feelings because it brings back happy times for me and I enjoy the whole process of making something.