Rule One: The Doctor lies.
Not just little white lies but big self-serving lies that get people killed, namely Amy Pond in “The Girl Who Waited.” While visiting the planet Apalapucia, the #2 destination for the discerning intergalactic traveler, Amy gets trapped in a faster time stream where she is hunted by robots that literally kill with kindness. By the time they come to rescue her, Amy has aged 36 years. The Doctor comes up with a plan to fold time and save their Amy before her long wait. Old Amy reluctantly agrees to help the Doctor on the condition that he saves her too. He accepts, saying “The TARDIS could sustain the paradox” but when Old Amy sees the Doctor standing at the door she instantly knows it’s a lie. She desperately runs to the TARDIS. The Doctor simply says “I’m sorry” and shuts the door.
Contrary to what he said earlier, the paradox would be too massive and only one Amy can be exist within the TARDIS. He tells Rory it’s his choice which one they save, which is really no choice at all. Rory protests, “This isn’t fair. You’re turning me into you.” But Rory isn’t the Doctor. In a touching split screen of Old Amy and Rory pressed up against the door, he breaks down and starts to unlock the door. Old Amy warns him, “If you love me, don’t let me in. Open that door… I will, I’ll come in. I don’t want to die. I won’t bow out bravely. I’ll be kicking, screaming, fighting to the end.” Helpless, Rory does the only thing he can. He tells her that he loves her. She tells him “I love you too. Tell Amy, your Amy. I’m giving you the days. The days with you. The days to come. The days I can’t have. Take them please. I’m giving you my days.” She backs away from the TARDIS and surrenders to the pursuing robots, which put her to sleep before she vanishes forever. When Rory asks the Doctor if he always knew they couldn’t save both Amys, the Doctor deflects the the question. “I promised you I’d save her and there she is,” he grins.
Rule 2: The Doctor practices triage.