That is not his motivation. Goku's motivation is not the defence of Earth, it never was. Otherwise he wouldn't engage in so many 1 on 1 fights when Gohan and Vegeta are capable of keeping up enough to not get in his way. His main motivation is testing himself and getting stronger and fighting alongside friends. Could be argued those are his only motivations. It just so happens that worse and worse threats keep appearing and threatening his friends, giving him the opportunity to fight those stronger opponents. His defending of Earth is a circumstance of his values, defending Earth itself is not a core value. Fighting, becoming stronger, and protecting his friends is all he does. He went to train Uub because Uub had the potential to become so freaking strong. He wanted to see how strong Uub could become, and to pit himself against Uub.