Complicated things for me tend not to be interesting. Simple things, like when it's really direct eye contact, or when you see a really beautiful shape, like when it's clean or well balanced in the frame.

If you give the same 200 frames to 200 different people, they'll all probably pick a different frame. The key is editing. Be intuitive enough to be shooting at the right moment, and when you're seeing the action happening just as you want it, shoot. And in the edit you have to go with your gut.