Obviously it's all genetics and background. You just have to work with what you've got. There are different areas to excel at in osu:
- Snapping/movement/cursor dancing (in the end it doesn't matter how you move, but hey it adds your unique signature to your replays)
- Accuracy (getting that perfect hit within the millisecond)
- Focus (allowing your mind to focus solely on playing)
- Reading (confusing patterns and overlapping notes, fast or slow AR)
- Spinning (clockwise/anti clockwise, consistently under stressful areas of a map)
- Endurance (maintaining focus for extended periods, playing for several hours a day without getting bored)
- Stamina (how consistent you are under stressful play)
- Aim (tiny circles, difficult angles, wide distances)
- Reaction speed (how fast you respond to sudden activity, can be overcome with memory or predicting the next note(reading))
- Streaming (consistency, speed, alternating, single tapping)
- Memory (how fast you can memorize something in the least amount of plays possible, how many things stay permanently stored in your memory eg. a difficult section of a map)
- Adapting (to sudden changes, a new input device)
There are probably a few others aswell. Most likely how you first start playing the game has an effect on how you will turn out. Training may help with your weak areas, but you won't be better than somebody who excels in certain areas no matter how much you "nurture" it. Also your input device will obviously help with the physical side of things, I won't even go into how many areas there are regarding input device.
Then you've got things like posture, ergonomics EG (A player sitting up straight will do better than a player who is slouching). The way you grip your input device, height of the desk you play on. How far away you sit from the desk, Computers fps and specs, your monitors refresh rate, how tired you are. The length of your fingers (may affect streaming speed), whether you sweat alot (grip). What type of headphones you have. How well your right brain co-exists with your left brain during play (this will have an obvious affect on confusing stream notes such as dj pops streams) etc etc etc
tl;dr: Nature.