Obama didn't wholly accept that. His statement that he'd "rather be a really good one-term president than a mediocre two-term president" reflects his disagreement. But official Washington and academia and public memory have come to associate success with reelection, so that's what Obama's trying for.
What Obama really wants at some deeper level is harder to say. Maybe he senses that the presidency hasn't really worked out for him, and some part of Obama doesn't want it. Maybe he feels in some semi-conscious way that he can't form a narrative of success out of his record. But that's not something he'd come out and say or even admit to himself.
Well that's the right thing to say under the circumstances, but fortunately for the country he has little chance of being either one.