Death is forever.
That's kind of obvious, isn't it. You can't die tomorrow, and come back next week. It's permanent. The people you love, the people who love you, will never see you again, should you die today.
I often wonder if people who kill themselves think it through. I mean really think it through. Because it's so permanent. Yes, death is a way to escape the pain of whatever you might be going through, but you can't come back once you change your mind about it. You can't give it a few weeks and say "Oh, I think I might go back to being alive again now".