Personally I wondered if my love for Brian made it easier for me to write from Justin's POV (not necessarily in the first person, but just viewing the story through him). Though I admire so much about Justin, it's Brian I'm technically in love with. Through that, I think I can relate to Justin's thoughts and feelings better, it's like I see Brian through his eyes.
Some of the writers I consider to be best at the Brian voice, seem to be Justinites (for want of a better word). It's just an observation I made when I was questioning the same thing as you, I'm going to read the other reponses with great interest. I'm glad you explored this question :D