He's gone from being by far the biggest fish in a good sized pond to being a tiny little fish in a massive pond. Who knows the ego's of players, perhaps he fancies being a big daddy fish again, being heralded as the finest player in the league rather than just a squad option in La Liga. Perhaps he thinks he is better off playing week in, week out with the freedom to express himself, rather than being even further restricted to minutes at Barca, missing the big games, etc... when he has a fight for his place in the Spanish starting 11 in the World Cup year, etc...
Things are not always so black and white as oh he's at Barca, winning trophies, why would he want to come back to this sunken little ship Arsenal and this is purely footballing based reasons, let alone any potential personal/family reasons.
