Negima Theories and Scenarios, Discuss possible future scenarios

My money's on something we haven't seen yet. I think that during the events of Ten Years Ago, he made a mistake—probably one that Gatou or one of the other older AR members warned him specifically against—that caused AR to get wiped out. Or at least he thinks he did.

Or it could just be survivor guilt. By all recollections, he's the only member of AR left who's still in one piece. Gatou's dead, Nagi's missing and presumed dead, Al's so messed up that he can't leave the heavy-magic area around the tree (I still hold to the theory that real!Al was killed during TYA and this Al is the last surviving shadow copy), and Eishun, Rakan, and Godel left before the final battle, from what we've been lead to believe.