insecurity.
There is some truth in this. I started lifting because I was fat and didn't want to be the fat kid at school anymore.
A funny thing happens though where after a point your priorities change as you leave school and the gym becomes the new environment you care about. Somewhere along the line many people, including myself to a certain extent, start to prioritise being powerful over simply 'being in shape' or whatever. I mean who cares if you have the 'six pack' you thought you always wanted if you're the skinniest weekest guy in the gym and you can't hang with the group of big guys who can bench/squat a shitload?!?
It seems at school lots of kids start off with a degree of natural muscle tone because they haven't discovered beer properly yet and they play sports regularly, but most guys lose that best ever high school shape they used to have. Once you start getting girls anyway and worry less about puppy fat and develop other interests being a kid with a 'six pack' seems less important than being a big strong man who actually looks muscular in cloths, or could stand up for himself in a situation, or whatever it is.
Sorry for the stream of consciousness, but to sum up, being big is cool if you're being honest with yourself about why you want it. For me it started off as insecurity but only up to a point. Once I was in good shape I started reading Flex and thought 'these guys are fucking huge, powerful and impressive looking'. It becomes its own thing away from insecurity, like a hobby or (arguably) sport of some kind. These days the skinny six pack kids from when I was 15 are small little guys, not a bad thing necessarily, but then neither is being big.
I know, 'epic insecurity post' or whatever, lol