For me it was moving to Portland, OR after a very unhealthy few years in TX and NY, and deciding that I was going to tie the new city to a new lifestyle in my head. So where "living in Dallas" meant "driving everywhere and eating fries with every meal" to me, "living in Portland" means "walking everywhere and keeping track of calories". That's just how it is there, no arguing with it.
In the bigger picture, though, it was realizing that I'm finally feeling ready to start a family with my husband, something he's been looking forward to for years.... and that I was running out of time to make myself a healthy person before that happens.