that's the external question isn't it? the one that everyone with depression, including myself is trying to find an answer for. if we knew we probably wouldnt have depression
the only thing I can suggest is when you are feeling down is to do as many things you enjoy as possible to perk yourself up. go for a walk, watch a movie you like, have a relaxing bath etc. And try to accept that your having a bad day, rather than fighting against it. instead of thinking "I really need to get out of this foul mood" try to think something like "today is bad, but itll be better tomorrow"
yeah I know easier said than done.
hang in there my friend, were all with you