For me, it had a lot to do with getting an international perspective. I was born into the church, but by about the time I was 10, we weren't going very often. When I was 16 and got my first job, I always worked on Sundays (and loved it, because when I family DID go to church, I didn't have to go).
When I was 18 I went around Asia for the first time and it absolutely blew my mind. My whole world up to that point had been the midwest USA (Reno, Denver, and Salt Lake City).
From there, I spent more and more time studying and trying to gain more knowledge of the entire world. I lived in Japan for a year during college (and I'm back here now), and that has also given me a lot of perspective.
Anyway, I don't have some "because of ____, I stopped believing", but I guess all I can really say is that once I realized how big the world is and saw the VAST variety of people, religions, cultures, etcetera with my own eyes, it just made me realize even more how little any single religion (in the classic sense) makes sense as the be-all end-all for the entire world.