in the fires of Pompeii the doctor meets Lobus Caecilius
CAECILIUS: Who are you?
DOCTOR: I am Spartacus.
DONNA: And so am I. CAECILIUS: Mister and Mrs Spartacus.
DOCTOR: Oh no, no, no. We're not, we're not married.
DONNA: We're not together.
CAECILIUS: Oh, then brother and sister? Yes, of course. You look very much alike.
then after saving him from the volcano at Christmas 2013 the time of the doctor he regenerates into Lobus face then in series 8 episode 1 Deep Breath he starts wondering why he choose this face
DOCTOR:Er, have you seen this face before?
DOCTOR: Are you sure?
BARNEY: Sir, I have never seen that face.
DOCTOR: It's funny, because I'm sure that I have. You know, I never know where the faces come from. They just pop up. Zap. Faces like this one. Come on, look at it, have a look, come on, look, look, look. (The Doctor pulls Barney over to look in the mirror on the ground.)
DOCTOR: Look, it's covered in lines. But I didn't do the frowning. Who frowned me this face? Do you ever look in the mirror and think I've seen that face before?
DOCTOR: Really? When?
BARNEY: Well, every time I look in the mirror.
DOCTOR: Oh, yes, yes, yes. Fair enough. Good point. My face is fresh on, though.
BARNEY: Er (Barney moves away from the nutter in the nightshirt.)
DOCTOR: Why this one? Why did I choose this face? It's like I'm trying to tell myself something. Like I'm trying to make a point. But what is so important that I can't just tell myself what I'm thinking?
then in series 9 episode 5 The Girl Who Died he starts to realize why he choose this face
DOCTOR:I'm not supposed to. Oh. Oh! (He looks at his reflection in the water and sees what he saw in the mirror back in Deep Breath.)
CLARA: What? What's wrong?
DOCTOR: My face.
DOCTOR [memory]: Who frowned me this face? Why this one? Why did I choose this face?
CLARA: Doctor, what's wrong with your face?
DOCTOR: I think I know why I chose it.
DOCTOR [memory]: It's like I'm trying to tell myself something.
DOCTOR: I think I know what I'm trying to say.
DONNA [memory]: (to Doctor 10) Just someone. Not the whole town. Just save someone.
DOCTOR 10 [memory]: Come with me. (A memory of Lobus Caecilus cowering as Vesuvius erupts in Fires of Pompeii. Played by Peter Capaldi, if you'd forgotten.)
DOCTOR: I know where I got this face, and I know what it's for.
CLARA: Okay, what's it for?
DOCTOR: To remind me. To hold me to the mark. I'm the Doctor, and I save people. (He shouts at the sky - the Time Lords.)
DOCTOR: And if anyone happens to be listening, and you've got any kind of a problem with that, to hell with you!
which made me think why did the Doctor choose the face of Lobus Caecilius? was it like reminder to save people what was it