He's gullible. Extremely gullible. Like, all a villain has to do is tell Goku that they're sorry and he'll instantly believe them.
But let's keep it 100, Vegeta (aka let Mr. Cell become perfect) isn't any better. He's even more arrogant than Goku is gullible. There's way more crap that happened in DBZ that was Vegeta's fault (for being arrogant) than Goku's.
I'll take a kindhearted protagonist over one that screws the Earth over because he's just sure he can't be defeated, despite taking more Ls than anybody.