thats like asking why did any ninja learn anything
He taught him because hes kind of his apprentice and he taught him probably by telling him all that he knows about the resengan, most notably how konohamaru used a shadow clone like Naruto does instead of just creating it like Jiraiya or Minato.
So he can become another Naruto. It surprises me that Konohamaru was able to learn the Rasengan in that amount of time, also being a kid. Compared to Minato, who was older, and yet took him a long time to learn it too.
that proves that the current generation always surpasses the previous one...and he learned it cuz he is an Uzumaki, i think, not sure though!
His surname is Sarutobi, so he's not an Uzumaki xd Yeah, but it's still a bit silly since that Jutsu is suppose to be really good and hard to learn. Now it's just like an mid-hard to learn jutsu (Or thats how it's made out to be since now Konohamaru can learn it easily).