I suppose this is the difference between clinical depression and sadness. When I feel particularly bad, I am often incapable of enjoying things. I don't seek out activities I enjoy as much, because they seem like too much effort for too little gain. I wind up in a rest mode that persists until the melancholy passes or some idea or event jolts me into productivity somehow. This productivity then makes me feel better about things, and when I feel better, it's time to relax and celebrate. I can enjoy things, so I'd better do it while I still can!
Knowing that happiness is temporary, and not something you can sustain by doing the stuff you want, is not necessarily conducive to health. :/