The short answer is William Shatner and my wife.
When I was a young tot (about 4 or 5 I think), I was lucky enough to be in that special zone where they were still showing reruns of Emergency! and there was this new show called Rescue 911. Yes, I realize I am dating myself here...
Anyway, after being absolutely glued to those shows, I remember telling a family friend "I'm gonna be a paramedic when I grow up!" Having NO clue what a paramedic actually did, except look really cool and save people. Mainly I just wanted to play with the defibrillator
Fast forward about 25 years or so... I was 30 years old and had a new wife, who really believed in me. We were going through a very fortunate time in our lives and she looked at me and asked me if there was anything in the world I could do, what would it be? I told her I had wanted to go to EMT school since I was a child, but had become an expert in making excuses as to why I couldn't. Her answer? Then do it. I will support you.
After 3 months of the most challenging classwork of my life (I always took the easy classes in college), I passed my class with one of the highest grades, and passed the National Registry on the first try.
I am in a fairly unrelated career that I love, and I will probably not do EMS full-time, but I have my application for a local volunteer department ready to go.
It's not as glamorous a story as some I have seen in this thread, but it is what it is. Whatever the motivation to serve, the fact that one has it is the important part!