The reason I left a cushy, high-paying job was that I think I can always get another cushy, high-paying job even when I'm older. On the other hand, my mid-twenties are years when I can afford to take (calculated) risks and do all the things I know I will have regretted not doing when I'm in my 30s and 40s and have more responsibilities.

I was also surprised how many people at and above my level in a big company seemed envious that I'd go to a startup.