A fighter trains to be a fighter or is he born a fighter?

This is a question I've been asking to myself a lot lately. In my experience there are people who are naturally good at fighting, but I've also seen people who sucked at fighting but after a lot of hard training they became the best around. So what does it take to be the best?, heart?, luck?.
When I started training MMA I was pretty good at it, even better than the advanced class guys, however I started with boxing and BJJ a few years back. But it was small-time training and I never sparred at boxing. Yet I was pretty good at MMA fighting, I guess I was naturally good at it, however as years went by and I kept training I got even better at it, actually to be fair I got about 300% better, If I fought myself a few years back with the same strength, height and weight I would kick the living crap out of my past-me. This makes me think, that even though I was already good at fighting with training I became better, and also I've met people who before training sucked at fighting and after a few years of training they where on the top-list.
So what do you think? is it natural? is it training? or perhaps things like heart, motivation, luck, etc. ?