I hear you, Tick. It's tough, man. And it is hard to process. I think the "why" is impossible to answer, and I'm sure you know that and are basically posing the question rhetorically. I have a great life. There is a lot that I find joy in. And yet I, too, have times where I feel pretty low and despair takes hold. I think most, if not all, of us do. It's just in how we deal with it. Some just aren't very well equipped to deal with it, for whatever reason. And it's sad when we see someone give in to that. I'm not at all equipped to address the deep psychological reasons behind it. But a couple of things come to mind.
The first is religious, so I'll just post my thoughts without veering too far off into P/R territory (although if anyone cares to debate it, we can do so in the P/R forum--please keep this part of the forum relatively free of P/R debates, please). But I think having faith and conviction in the existence of God and his promise of an eternal afterlife is a big factor in helping a lot of people pull out of those feelings of despair. There is joy and hope in something FAR beyond and more lasting than whatever trials of this fleeting 80-something year life (or thereabouts). Without that, yeah, I can see it being far harder to pull out of that depression cycle when it feels like all is lost and not seeing a way out.
The second has to do with celebrity. I'm not saying suicide is a uniquely celebrity thing. It isn't. But in the case of celebs, I see a pattern of living a lifestyle that more or less loses tough with the day-to-day humanity of leading a "normal," regular life. I think that is especially true of those who attain celebrity status at a young enough age that they never really learn what it means to lead a normal life, whether musicians, actors, athletes, or what have you. They often live in a very artificial world. And I feel like that robs them of developing some of the coping skills that are a very healthy part of life.
Anyhow, for what it's worth, those are my thoughts on the subject. Good topic, Tick. I feel for you.