At this specific point in time, he just knows how to play the game better than the other two.
When I wrote my Ethan essays I made myself look at things from Ethan's point of view. Justin has really described Brian in very unflattering ways. Justin wasn't very convincing in 216 at telling Ethan that he was fine with his boyfriend not being into birthdays. Here Justin is telling Ethan that his boyfriend won't let him get a cat he shows an interest in. From what Ethan knows Justin is unhappy in his relationship with Brian. As far as we know (and we seem to know most of it) Ethan has no clue how happy Justin has been in the past with Brian.