Like anything on reddit, it's a complex issue that people want to boil down to one factor for a snarky upvote-gathering comment.

I have absolutely no doubt that genetics play a significant role in the Rock being who he is. 

Disclaimer I'm gonna talk a lot and site nothing.

There's a lot of variables in the brain. I mean, my understanding is psych students learn too much dopamine causes schizophrenia and too little means you're ADHD. But it isn't, like, three positions on a switch. It's a scale, and ten people might fall at different levels. Two people who would never be diagnosed with either can be at opposite ends of the 'normal' segment of that spectrum.

So when you get to food. There's a reward system at play, that's apparently proven, and a scale to that too you have to figure. Some people, an occasional pie feels like a reward, but too much feels like overkill. Some people could eat a cake every day and feel great every time. And to some a slice of pie every week is just meh.

Exercise has a reward system too. And people want to boil it down to will, that the next mile just takes willpower. But there's different reward systems. The third cupcake brings different levels of happiness to the obese person, and the third mile brings different levels of happiness to the fitness freak.

Again we're boiling down numerous factors to one. I mean, people with identical reward systems can get wildly different results, and there's certainly people who enjoy food more than a hypothetical other, who end up more into fitness because they value health more. I'm not preaching life sentences or fates over here. But genetics is a million variables we still, in the era of supercomputers in pockets, struggle to fully comprehend. In 3000 years if humans still roam the earth I'm sure we'll find that people who've made fitness their life, like the Rock or LeBron who got enough out of sports to chase them even when a multi-million dollar house wasn't a guarantee, had a good bit more fitness reward system in them.

As another disclaimer and re-iteration, although I've painted food and fitness as identical reward systems to talk genetics, there's obviously a healthier option, and if you want to be healthier you can do it. Again, many variables.