Gohan's always been a good older brother to Goten, even a sort of father figure, considering their age difference. After Gokuu died he probably took it as his own responsibilty to raise Goten as his Gokuu would have. It's surprising that Gohan waited so long to train Goten, but I suppose he would have expected Chi-Chi to get mad at him, considering her response to Gokuu's attempts to train Gohan as a boy. Gohan was completely taken by surprise when Goten showed him his strength, Gohan hadn't really expected his little brother to reach the level that he had in so little time. (Then again, it took Gohan several years of training to go Super Saiyajin, so he had every right to be surprised).

Gohan does tell Goten off sometimes, (never Goten's fault, of course, he just happens to keep finding dinosaurs... uh... yeah), I guess that's the result of the fatherly role he was trying to play.

Goten has a really high opinion of his brother, always supporting him, (calling him cool way too many times...) and generally looking up to him. He seems to respect his brother's judgement, if Gohan tells him off, he goes submissive and does what he's told. He really does look up to him, like a role model. Goten seems to be content to follow in his brother's footsteps in everything except on an academic level.

It's probably just Chi-Chi going easy on him, but Goten never ends up being as pampered and heavily educated as Gohan was. Goten didn't have to go through as many traumatic events as Gohan did, and as a result is a far more relaxed kid than Gohan was as his age. Whatever happens, they've both inherited the great Son family cluelessness... and cuteness.