I think everyone has their own motivations. It seems like 90% of all the people in my gym are motivated by vanity. Like those people who will all of a sudden get a membership, gain 15 pounds of muscle in two weeks, and scream for the whole gym to hear him during his sets. All so he could wear a sleeveless shirt at the club and look good in the thumbs up pose.
It does psychologically comfort me to look good and all. For that aspect of it, it's about customization. I like to be able to customize my meat mobile. Sculpt it to look a certain way. And feel a certain way, too, and that's another thing. For me, it's more about feeling good. I like to be able to feel healthy, lively, energetic, and of course strong. It gives one the incentive to live a healthy lifestyle.
Other reasons I'd say is for it's discipline and meditative benefits. Working out is also great because you can have a shitty day but go to the gym and take it out on a back workout and feel like all the day's shittiness was insignificant.