I think you're asking the right question. Very unpopular opinion, but I think some of it comes down to genetics. I know a guy who is just always happy, no matter what. I've just never seen him feeling down. He grew up in a somewhat challenging situation but it doesn't seem to matter. I really think some of it is just the way he's wired. He has a lot of energy, is physically active everyday and doesn't fall asleep easily. This dude has the personality of a friendly dog (in a good way). He's a very reliable friend and I would trust him with my life.

The complete answer is probably that it goes both ways. People have a natural tendency to be more anxious or more energetic or happy or sad, a baseline if you will. If you take care of your body, it helps. If you're exposed to stressful situations, especially in childhood, that can become a risk factor later. But, IMO, there's definitely people out there who are genetically lucky in this area, and others less so.

I'm going to note that just suggesting that there's a genetic component is controversial because it goes against the modern day idea that "you can do anything if you just try hard enough" a little bit. The fact is though, we're all genetically unique. I think it's a good idea to acknowledge that we all start in a different place. That doesn't mean you can't work on yourself. Even short people can run a marathon and develop a lot more endurance if they train as opposed to playing videogames all day.

It also comes down to knowing your strengths and weaknesses. I feel like I have a natural tendency to feel more down and tired than others. I've developed healthy coping strategies and structured my life in a way that I can minimize stress. Being aware of my own limitations makes me better able to cope with them. Pretending that I'm one of those always happy people would not.