What if a deer was one day given the power to transform itself whatever it wanted it to be?
So the deer got the head of a lion. Then it wanted the wings of the eagle - and got it too. Now it wanted the magnificient tail of a lizard, and developed it. Then it covered itself with the fur of American wolf, and got the gorgeously colorful lips (beak) of a Macaw [that was fixed on the head of the lion].
Finally, it desired the legs of kangaroo, and finished the production with the vinyl of a jaguar and the dangerous claws of Amur tiger.
I do think one becomes what one intends to be. I think it starts before one is born. This guy in the picture feels good about himself you can tell, but it comes with an inner feeling that is kind of indescribable, unless one knows the benefits of exercise. Exercise affects the psyche in a mysterious way. There is definitely a body-mind connection to your question.
I notice the kids at school are very tuned in to being what they expect themselves to be. More so now than when I was in grade school, because they have more freedom along with clothes and shoes that allow them to move freely.
Also junk food and lack of exercise takes away motivation/expectation. Then you become a slug and you don't care. You believe that is your reality. Sometimes people have accidents, and even though they recover physically, they stay in a habitual self-protective mode. They loose belief in themselves and stop expecting themselves to be strong and capable. This mind set becomes their reality.
I used to be a kind of a bit 'exercise freak' myself. The trouble with bodybuilding is that one can not turn back after they get a superman body. A sleek person may develop their body if the need be - but it's difficult to get sleek after one has developed a lot of muscles - can not get the actual chassis, to be honest. And that's kind of a problem. It's something like a tattoo that you do not like, and can not erase either. But it does impress some folks who do not know the reality.
What's more - bodybuilding doesn't necessarily increase longevity.
Its a complicated question once you try to answer it. I would say many things affect how we come to believe/think how we should be. Up-bringing is one. We watch our parents and pattern ourselves after them… in our own way. Environment is another. There are kids who expected themselves to become singers after watching a couple seasons of American Idol. They got their parents to buy them microphones and keyboards. And the parents, of course, encouraged them. (If they could've they would've themselves.) Eventually, these very motivated kids became really good through lessons and practicing with music teachers and voice coaches, etc.. Sure enough, some of them got onto American Idol as teens! So, not only environment, but encouragement from parents, coaches and teachers. And not only that, but your surroundings, such as what type of neighborhood, state, and country! Maybe the most important influence is the peers one goes to school with and the friends one makes/chooses in early life. It is also important to consider what you want to be when choosing a life partner.
Perhaps this comment does not make any sense: but every good actor is aware that there is a lot of logic behind even the most minute actions and gestures of a certain, given cinematic character. There is a lot of reason why an army officer doesn't tie his shoes the way a homeless man does. From a certain view, life is as precise as mathematics.
~ oopsy I deleted the post, your message didn't show up till then (I felt it became unnecceassary because the original question was gone (edited). But because you took it seriously, I put it back again!