I'd always been on the skinny side and was only about 5'8" until my senior year of high school, at the end of which I grew to about 6'even. I still didn't put on weight, despite being a 3 season athlete. Throughout college I worked out sparingly at most.
Then after a particularly tough breakup with a girl I was really into (how many times have you heard this story), I started working out at home out of spite and to relieve stress. Then, after seeing how I looked in the mirror right after working out as opposed to how I normally looked, I decided that I should join a gym.
I joined the local Planet Fitness because I've always been a little self-conscious and I liked the no-pressure atmosphere. I started going 5 days a week, loving every minute of it. Now that I've started law school, I'm much more limited in my time, and only make it 2-3 times per week.