Yeah, I agree that Justin's too stubborn to have just let it end there. He fought through the entire series to get Brian to settle down with him, but I think once he got Brian's affection Justin wanted more. That's why he left when Brian said he wasn't going to propose or anything. Then when he almost died and Brian said "I love you" and proposed, Justin got what he wanted. But it wasn't the Brian he fell in love with, it was the Brian he tried creating, and Justin didn't like it.
BRIAN WANTED TO MARRY HIM, OK?!
HE REALLY DID, BUT JUSTIN WAS CONVINCED THAT BRIAN WAS CHANGING, SO HE PUSHED HIM AWAY, AND BECAUSE BRIAN ALWAYS THINKS THAT HE DOESN’T DESERVE JUSTIN HE AGREED AND LET HIMSELF BE PUSHED AWAY
BUT HE REALLY WANTED TO MARRY JUSTIN