I did martial arts as soon as I could as a child and stuck with it through early high school. We didn't get a wrestling team at my high school until my senior year, so there was no natural transition for me to keep going. I really wish there had been because I graduated in 2005, right when UFC's The Ultimate Fighter came out - what could have been. At that point, I was also getting burned out on martial arts - partially due to doing it for so long, but also due to being put in the adult class with fully physically developed men in their 20s and 30s when I was only 13-14-year-old. I was a late bloomer physically and it's really discouraging to go get destroyed a few times a week when you're still just a kid. But it did make me quite tough, gave me the confidence to stand up to and back down some of the kids who thought they were real tough shit at my school, and gave me a reputation as one of the baddest dudes you didn't want to fuck with.
I stopped doing anything athletic by my junior year of high school and that continued through college, where I got fat. I knew things had to change when I developed stretch marks on my abdominals below my belly button. 12 years later they're completely faded, but seeing them fresh was the shock I needed to change.
I can honestly say I got into lifting weights purely for bettering myself. I loved comic books as a kid, have always watched pro wrestling and fighting, so those images of idealized, muscular physiques undoubtedly play some subconscious role. But I really did it just to get myself in shape and have a new outlet where I could always try to do better each day - 1 more rep, 2.5 more lbs., etc.
All these years later, I still really do it just for me. The girls are cool, the comments you get in public are cool, everyone loves having their ego stroked - but take all that away and I'd still train and diet just as hard. Bodybuilding is what I do and a bodybuilder is who I am. It doesn't 100% define me as I have other interests in life, but it's been my primary focus for years and will be for the next 5 or so.