I even got the impression from Tennant and even Eccelston's regenerations that they really were not quite sure they'd actually regenerate. Keep in mind, the 10th Doctor had the Master die in his arms, begging him to regenerate and he didn't. So I think honestly ever since the 10th's regen into the 11th it's been something of a surprise. Granted yeah he held it off, but I think a part of him was like 'I don't want to go, I don't know if I'll make it this time.' and given the overly violent nature of his last regen, with the Tardis crashing and getting massively trashed, it wasn't something that he thought 'Eh, I'll just turn into a new me and that'll be that, it's usually peaceful." This was a scary as hell thing. So in many ways I think he's already got that fear of mortality kinda thing, he can live for a very long time, but each time he 'dies', it could always be his last. Especially being the last true time lord out there, when he's gone, he's gone.