THIS CONTAINS A LOAD OF SPOILERS!
READ AT YOUR OWN RISK!
After months and months of waiting, it's finally been revealed! Why did the 12th Doctor choose that face? Where had he seen his face before? All questions were finally answered in S09E05 'The Girl Who Died' on 17th October 2015.
In 'The Girl Who Died' we saw Vikings, The Mire and Odin. Not forgetting the Dragon Puppet! But the key moment was, we saw Maisie William's character (Ashildr) die. The Doctor (Peter Capaldi) and Clara (Jenna Coleman) doing their plan to stop Odin, but forgetting about Ashildr with the Mire's helmet on. The Doctor takes it off and notices she has died.
The Doctor reacts as if there is nothing he can do. She's died and he can't do anything. But whilst him and Clara are in the Viking building, the Doctor freezes for a second with shock. It then sprung to mind why he chose that very face of his. He looked in the bowl of water and saw his reflection but not as he thinks. He saw his past face from S08E01. Which was 'Deep Breath'. That is where he first mentioned that his face looked familiar. But why?
The Doctor is amazed! This when we had a flashback! But to the place we've all wanted to see return. Pompeii! We had a flashback to S04E02 'The Fires of Pompeii'. At last! Fans got excited as we saw David Tennant and Catherine Tate again! The Doctor remembers saving Lobus Caecilius and his family from the volcano erupting and was going to destroy Pompeii.
When the 11th Doctor (Matt Smith) regenerated into the 12th Doctor, he chose that face as a reminder. A reminder that he has to save people not matter what the consequences are! This was when the Doctor went back to save Ashildr using the Mire's Medical Kit. The consequence here, she's likely to be immortal now. Just like Captain Jack.
Now the Whovians of the Whoniverse know why he chose that face. Peace at last.