My reasons and thinking have changed and evolved since then but I thought about since I first learned about death in general, so about 6 or 7 years old.
My first thought was that people could die in so many unpleasant, upsetting, or horrible ways, that killing oneself could be a way to avoid that.
The one that followed was born from observing people live unpleasant, upsetting, or horrible lives and that I wouldn't have the courage to live that kind of life and so I would kill myself to avoid it.