He didn't want to leave Arsenal, he has said it before. This time it wasn't the club that sold him as well, it was Wenger. His attitude was poor, he was ruining the mood in the team as all he was doing was boasting about how Barca wanted him and no-one else at Arsenal. He also turned up late to training and was generally a nightmare. He really angered Steve Bould and Wenger.
I don't see why he won't like Barca, he'll get to win trophies and be part of the best team in the world, he must've known that there wasn't really any way he'd displace anyone from that incredible midfield. As I said earlier though, he'll destroy that midfield if he plays there, his passing is fairly poor and he holds onto the ball for far too long. CB is a good spot for him.
Glad to see him gone tbh, we are stocked in midfield already with Wilshere and Rosicky coming back. We don't miss him at all, Wilshere is definitely better and Diaby is proving that if he stays fit he is a better physical option in midfield than Song.