The truth is, the more emotion they express, the more powerful they are. Look at Gohan, for instance. He is only half-saiyan, yet as an 11 or 12 yr. old he was strong enough to defeat Cell. That is much more than Goku could say. Then, there's the carefree Goten, who was a Super Saiyan at a very young age. I agree with the emotion thing. Because Gohan and Goten are half human as well as half saiyan, they have the ability to hope and love, yet they still have the strength of the saiyans.
So, since Goku has lived as a human for most of his life, he might as well be part human. All Vegeta cares about is his ego, which makes him arrogant, and takes away that boost of strength that always seems to come to Goku in the time of need.