This may sound high, but I've felt this is something that makes a lot of practical sense. This is how I think it really works:-
1) The more you help others, the more your expertise grows, simply because of the breadth of problems you get to tackle
2) The more you help others, the more you get it in return (not that people keep track, but, what goes around, naturally comes around)
Consider this - 5 out of the world's top 10 paid footballers are midfielders.