I touched on this briefly in this piece and I discuss this in more detail in other articles I've written, but I transitioned because my gender identity didn't match my biological sex. I just took longer to move forward with transitioning because:
1. I had always assumed it would cost millions of $$ and only wealthy people like Chaz Bono could afford to do it.
2. I feared needing a lot of surgery in order to transition.
3. I didn't have the crippling dysphoria, so I wasn't as motivated as some transgender people to seek relief through transition.
Once I discovered that the majority of the transition for me would be accomplished merely through taking hormones, and that it wasn't as cost prohibitive as I'd feared, that's when I began to seriously look into transitioning.