Thats good question. It was first mentioned as you said in Forever Young scene, and I dont know the exact answer how it came about.

But I suppose Brian started to consider himself somewhere during season 1 as Justins "teacher (of course not literally)", as someone who introduced him to gay lifestyle and although he was denying externally and especially in front of others that Justin is his responsibility, he cared about him and did everything so that Justin could develop all his talents and become successful, independent and satisfied without being negatively influenced by the society because of his sexual orientation. He knew Justin is especially gifted but even those people can sometimes fail under adverse conditions.
So it seems to me that in the Forever Yound scene Brian just verbalised how he felt inside about Justin and what he wanted to for him.
It was motto for his own life and in Justin Brian saw someone with even more potential than he had, and combined with affection he had for him, he considered it as his mission to help Justin in every way possible.