I used to be a serious alcoholic and occasional (hard) drug user. The alcoholism progressed until the point where it got me in trouble with the law and quite nearly killed me. My mom spent $200 she didn't have on natural detox supplies and I quit instantly (which is incredibly painful. Don't try this unless you're as crazy as me) and I just fucking quit. It hurt like hell. Hell, I tell you.
I remembered my weight training from high school baseball and how much fun that was, how much focus it demanded....so I started doing pushups. I did a shit ton of bodyweight work during my "recovery" from withdrawls, then I kicked the weights in around early-mid 2012.
My motivation is part competition, part the fact that I like looking good, part an act of love towards myself (<----this is one that NONE of us really focus on, but it's true) and part a way to exercise control over myself.