It wasn't any one event for me, but the combination.
Friends - Have two of them who are about 6' and each weigh near 300lb. They also smoke, breathe heavy and I thought to myself I'll never get that big.
Family - Grandma died at 59 and my father at 47. I wanted to take a proactive life in my health since. For her it was preventable, but smoking earlier and working in a bar left a lasting health impact. For him, it was complications from renal failure. How much of it could have been prevented? I'm not so sure, but 20 years on dialysis / transplant was much longer than the average. I'd like to think I'll live into my 80s. Have to take certain actions now to make that a reality.
Other people - Lots of smoking hot guys out there. I was the most fit when I was single, trying to impress someone subconsciously perhaps. Now I just want to look good mostly for me. If someone else happens to notice online or in person, more reinforcement to keep at it.