For me, it was watching my uncle slowly drink himself to death. He hated himself so much (and we still don't know why,) but instead of getting help or ending his life quickly, he spent the last 10-12 years alienating the family and making it as difficult as possible to be around him.
And I could sense that same level of self-hate in me, but I also could never imagine offing myself. I just got so damn tired of swinging between being 'OK' with my body, and then so disgusted by it that I would avoid showering or anything else that would require uncovering. What had always stopped me before was thinking about every detail of transition, how it seemed like this impossible (and expensive) task.
Then I found out that I actually did have access to therapy and HRT, which I considered the biggest initial step. So I had a choice - chase a possibility at happiness, or continue down the road my uncle took.
So here I am! 🙂 It's definitely hard at times (not to mention a lot of awkwardness,) but I am already SO much happier than I was even a year ago.