That was my initial assumption when he took a hiatus to direct.. but I was hoping he'd re-up for a little while longer.
Hard to blame him to leave now, he's been doing the show for 16 years, at some point you might just get tired of the same routine, no matter how fun it can be.
Personally, I'd guess he might need a new challenge, and directing is the next logical step. He sees his friends and former co-workers going on to bigger and better things, Carell is nominated for an Oscar, Colbert is taking over The Late Show (which I originally thought Stewart would takeover). I was bummed to see Colbert leave because I was such a fan, but at least I still had Stewart, it'll be weird without both of them on TV.