Is there such a thing as thinking about John too much?

Definitely not, especially not when he's being good-parent-John and discussing Kurt with Sam, rather than dismissing him.

Sam tells Dean in Fresh Blood that he's been following him and looking up to him and trying to be like him since he was four years old - looks like he started that even earlier, even if Sam can't remember it. Being a "big brother" is obviously the most awesome thing one can be. And if one needs an imaginary African lion to be one's little brother, well, so be it