Most people would say they want to live because of various things they enjoy in life, and they feel like anyone who's suicidal just needs to find something they want to live for. What you can't understand until you've been there is that when your brain chemistry is so out of whack, there is absolutely nothing that seems worth living for. Future pleasure seems not worth the risk of future pain. I've actually had someone tell me he doesn't want a better job, or a new girlfriend, because at some point he'd lose the good thing and be hurt again. So he wouldn't enjoy it if he got it because he'd just be waiting for something bad to happen.
I never understood this until I was near 40, and hit some kind of mid life crisis which led to a deep depression. I didn't try to kill myself but I really didn't see anything to live for either. I remember going on a road trip and driving carefully on the ice, then realizing the irony of that. So I asked my passenger, "You don't care if you die today right?" She said "Right." I said "Okay, then we're driving 90 all the way home, fuck it." Got into a spinout and almost went under an 18 wheeler but I pulled it out in time. LOL. It was an interesting trip.
Anyway I'm not sure where I'm at now. I have gotten back the "wanting to live" feeling even though objectively I have even less to live for than I did before. It really is brain chemistry, and perception does not always correlate well to reality.