If I ever figure out what causes this

I will be a very wealthy man! I often sink into deep despair, not really suicidal though I wouldn't be against dying just not willing to physically cause my own death (or anyone elses). The weird thing is that there doesn't seem to be any rhyme nor reason to my mood change. Nothing triggers it, I just go from being in a reasonably good mood to suddenly not caring about anything. (Obviously it just occured and I feel the need to talk about it)