Everyone who fosters has a different reason. Some were foster kids themselves, some know a specific kid who is entering care, some just want to save the world one kid at a time. I probably fall into that last category.
When I was in high school, I had a Huge Crush on a friend who I learned was in care. (It turns out he's gay.) He was perfectly normal, smart, and a few years older than me so I worried about what was going to happen when he turned 18. (He was normal, but his foster brother who I also had a bit of a crush on, I think only because another friend had a crush on him, was a little less so.) I've heard from other friends that he went to college and hasn't had any crash-and-burn moments, but can't find him via Google.
For a while, I was telling people that my experience with Foster Friend was why I was interested in being a foster parent. I did suggest to my about-to-be-empty-nesting parents at the time that they should be foster parents.
But then I remembered the real, and much more meaningful reason. I have no idea where in my memory it was hiding.
When I was in college, I volunteered at a home for young kids--early elementary--who had been removed from their homes but didn't have foster families. It was a clean but institutional place. After school, the eight 6-year-olds all sat on the tiled floor and watched TV. I would take two or three aside and read to them. They were heartbreakingly adorable. And it killed me that they didn't have families to live with. I even thought about how I could manage to move off campus to become a foster parent right then and there.
Obviously that didn't happen. But I've always since then known that I would someday be a foster parent. And now that's finally about to happen.*
*The licensing process--and the ridiculously long time it is taking for me in particular--is another story all to itself. Stay tuned.