I have taken to the idea that people have a right to die.
The biggest argument I've heard against suicide is that it hurts other people, presumably by causing them a sense of grief and loss. But when someone chooses to move away for a long time, they also cause a sense of grief and loss, yet their friends and family believe it is their job to accept this loss because they love the person moving away, and want to let them do what they think is best for themselves. So when someone says "I've decided to commit suicide", it seems like their friends and family should say "aww, we're really gonna miss you! but we support you in your decision."
It also seems like cruel and unusual punishment to keep someone alive who wants to die. In fact, it is blatantly thought of as such in movies when the heroes rush in to the bad guy's lair as they are trying to commit suicide — because the bad guy knows they've lost — and the heroes prevent them from doing so with a line like "we're not gonna let you get away that easy!" That is, we think of life as punishment for this person. And it is. And it's cruel and unusual.
Also, we all do things that might kill ourselves, or reduce our life expectancy. We drive. We live on fault lines. We drink alcohol. We do challenging things that cause stress. We're not always optimizing for length-of-life, and it seems fine that we don't. We don't actually care about long lives as much as we care about great lives. Many influential historical figures committed suicide, or died pursuing their interests, and in any case, we don't rank historical figures by how long they lived but by what they accomplished.
And sometimes accomplishing great things is dangerous. And sometimes it alienates people from the society they live in, and they don't want to live there anymore. And sometimes the source of great inspiration in the mind is also the source of great depression, yet we cherish artifacts and knowledge drawn from such sources even though they may have ultimately led their creators to kill themselves.