I can think of two reasons.
First reason: because things can and probably will get much better. Joy in life comes from the little things. That sounds cliche but it's true. If I could talk to my 14 year old self, who was severely depressed to the point of trying (and thankfully failing) to take his own life, I would tell him about the next 20-ish years. Even though much of it will be hard, it will still be good. And he will grow in ways and get to experience things that he can't even begin to imagine. That's one thing I'm glad he failed at.
Second reason: because believe it or not, you will leave a giant crater in the life of someone (or multiple someones) where you once existed My great grandpa hung himself in 1929. That's all I know about him aside from his name. I never met my grandpa (died of cancer) but I remember my dad telling me a little about the impact it has on his dad, who was about 15 at the time of his father suicide. Long story short, my grandpa basically stopped growing emotionally at 15. He was a teenager who was very suddenly thrust into the role of an adult.
I don't know what was going on with my great grandpa that led him to take his own life. I do know that what he left behind was a disaster. Including three generations of trauma, manifesting itself as a cycle of physical, verbal, and emotional abuse. He effectively destroyed his children who proceeded to pass that destruction all the way down to me.
If you've never watched Ted Lasso which I highly recommend, one of overarching themes is Ted's difficulty dealing with his father's suicide, which occured when Ted was 15. It's a light hearted show overall but there are a few scenes that really hit right in the feels.
Even if you don't have kids, there are people who's lives will be permanently altered for the worse by your untimely death. Some will blame themselves, wondering what they could have done to prevent it.
