To be honest I can't really recall one particular event. I started as a teenager 15 years ago, experimenting with Flash and Photoshop. I was amazed at the possibility to create something out of nothing. Of course I did a lot of personal projects, everyone will tell you to practice a lot, and you should.
But most importantly, I think I've always been a dissatisfied person. Design, as I see it, is about fixing stuff. Making things easier to use, to understand. Turning corporate messages into something meaningful. Or fighting corporations when they deserve to be fought. Telling the truth to clients and our clients' clients.
I remember this t-shirt I used to wear when I was working in a big agency, in Prague : Design is honest, Advertising is lying. Of course it was provocative, but it sums it up pretty well I think. During my years of design work I regularly asked myself, am I doing good design here, or am I just selling the stuff I've been asked to wrap nicely? When the answer was the second, it was time for me to move on.
Did design change my life? Maybe. It taught me that I can speak up for what I think is right, that a nicely designed idea is powerful. I've been sued for websites I made, for articles I wrote... Really it's a lot of fun.
I cannot tell if I'm happy with my career because I'm a dissatisfied person. Truth is I don't care. My career is pointless. What I care about is to make great designs and fix stuff. This world needs it. Being a designer is a burden.