So in the first episode of this season the Doctor asks about where his face came from multiple times, and he says something along the lines of “It’s like I’m trying to tell myself something”.
As we know, Capaldi played “The Fires of Pompeii”. He was the father of the family that 10 saved, after Donna convinced him to go back.
So my theory is that the Doctor gave himself Capaldi’s face as a reminder to go back and save Gallifrey, a society that is about to be destroyed (just like Pompeii).
It’s just something I thought up, it doesn’t explain anything and it probably isn’t even remotely true, but I like it.
"The lock on your diary wasn’t very good, so it’s your fault I read your diary."
"Hey babe, here’s a flash drive with some sexy photos on it"