I have multiple reasons. For one, I'm all alone. And that's literal. Lost all my family over the last 5 years - mother, sister, multiple cousins - with my mother being the last to pass. Had a very small family and I'm the last one remaining. Second reason is I have no friends. So, when I say I'm alone, again, that is literal. I'm not married, haven't any gfs, and I have no children. Not the best place to be in life and I want no part of it.
Third reason is not looking forward to aging and want no part of that, either, especially when I'm all alone. That's not a good place to be with no support whatsoever. I don't want to have to deal with the miserable diseases, and the like, that come with aging. I'm 59 now, and the horror movie ending for me is ending up in a nursing facility somewhere, with no family or friend support, needing to have my ass wiped by someone, and the dribble removed from my chin after having been spoon fed by some aide. No. Not what I want. Hopefully, I get the hell out of here before something bad, like a heart attack, or stroke, turns my worst fears into a reality.
A forth reason is that at 59, I can no longer, realistically, pursue that which would bring me happiness. Take out of that what you will, but that ship has sailed quite some time ago and I just missed the boat.