"Why's Goku always one step ahead of Vegeta?"
"Cause Goku fights for his family and friends, Vegeta only for himself."
Ya know, reading that reply made me realize how that is so not true at this point. Goku's never really shown any particular attachment to his family at all. Sure he cares about the planet's safety, but the family argument, I can't really see it. Vegeta on the other hand, over time has shown he actually cares about his wife and children a lot.
A good example of how awful Goku is as a family man and how much of a better one Vegeta is, would be when Bulla was about to be born. When Vegeta was adamant about staying with Bulma and not leave her alone, Goku was all "Oh who cares, let's go far away training, she's the one to give birth after all, not you! Look at me, I was dead when Goten was born, hihihihihi!".
Damn, just realized that I don't really like Goku anymore. LOL.
But anyway, the real answer to the question is that Goku's the protagonist, and that's all there really is to it.