Being given 3 years to live by my doctor.
I was massively over-weight, diabetic, cholesterol out of control, kidney function impaired, and liver enzymes hinted at damage there too. 1 year later, I was able to drop all medication after losing 145 pounds. At the beginning of that year, I began doing body weight exercises, starting out with push-ups on my knees because I was too weak to do real ones. I progressed to doing real ones, and unweighted squats and some curls and over head pressing with some dumbbells that were laying around.
After 6 months of that, I was much healthier than I had been in years, since high school sports really. I joined a local gym and got started on a circuit training program put together by a trainer there. The guy knew his stuff, and even though I don't always agree with his programs, he taught them well and genuinely cared about his clients. Then one day I ran across a good forum, saw some of BtB's workouts and tried my first deadlift. Nothing has been heavy enough since.
Fast forward to now. My waist is within an inch of where it was when I weighed 215, and I'm around 250 these days. My blood work is stellar. I'm getting stronger. My mind is sharper then ever before. This is what continues to motivate me. To be stronger, smarter, tougher. To be the best. Its a long way from barely being able to make it up the stairs to my office.