I had said this a little while back and everything that is coming out since seems to confirm this. Brown appears to be looking to be traded. He believes that he can be the #1 guy. He believes that he has been sacrificing for the good of the team, prior to this season.
I like Brown, I think he is a great player, maybe not quite as good as he seems to think he is, but he is a very good player. He is coming off his best statistical season, his value should be high. No reason not to test his trade market.
As to the Celtics, I suspect that they don't want to trade him, that they prefer to keep him, at least at this point still. It can be tough, but not impossible, to trade a guy like Brown and get full value back.