“I am not a good man! I’m not a bad man. I’m not a hero. I’m definitely not a president, and no, I am not an officer. You know who I am? I… am… an idiot. With a box and a screwdriver. Passing through. Helping out. Learning. I don’t need an army. I never have. Because I’ve got them, always them, because love is not an emotion; love is a promise, and he will never hurt her… You didn’t notice, did you? While you were doing all your silly orders, while you were showing off, the one soldier not obeying?”
— 12th Doctor, Death in Heaven
The important part of this quote comes towards the end. But you need to know a little something to fully understand the impact of the quote. You see, one of the characters had been turned into a Cyberman. And Cybermen are “upgraded” to have all emotions removed. They are no longer themselves, and they are to obey the head Cyberman. And the head Cyberman wanted the other Cybermen to threaten and perhaps kill The Doctor and another character. But one of the Cybermen loved that other character. He loved her a lot.
Now, the Cybercontroller thought that they had everything covered because Cyberlover had his emotions erased. But the Cybercontroller was wrong. Because, as the Doctor said, love is not an emotion. It is a promise. And before that Cyberman was “upgraded,” he promised that other character that he would protect her. And he didn’t forget that when he became a Cyberman. He remembered. And that promise made all the difference.