He's going to shine more in a Bremen team that is obviously inferior to Arsenal, get more minutes, more or a run in the team etc etc. Maybe he just feels more comfortable back in Germany.
But us fans only get to see the rare glimpses of game time, you'd think our coaching staff would see enough capability across all the hours of training to mark him out as the next Bellerin-type success story, albeit in a harder position to claim on the field. It just took a brave decision to decide that he offers more than Ramsey and Chamberlain, and Wenger didn't take that.
By the end we wanted to keep him right? so that says enough. I suspect he was monumentally pissed off at first being loaned to West Brom and second doing nothing there, so that made his mind up and quite right. Parked at Werder Bremen for 1 year, or 2 years, before Bayern come calling.