Alright, so this entry might be the one that results in a thousand comments insisting that the Doctor and River Song have fantastic chemistry and are a great couple together. And although maybe on the surface, people might be able to see chemistry (although even that's debatable), if you dig any further beyond there, the relationship falls apart pretty quickly. Being with River turns the Doctor from someone who is pacifistic almost to a fault into a man who is turned on by violence. There's something wrong with that picture. River Song isn't so much a character as she is the Doctor's shadow -- through very little fault of her own, everything in her life revolves around this man. She was raised to kill him, she's in prison for the supposed murder of him, her parents are his companions, even her archaeology degree was just a way for her to get closer to him. She's a character with zero agency and nothing really going on her life other than her occasional meetings with this man. And the really sad part is, there's no way for him to ever feel as deeply for her as she does for him. He is her life, but she is just a person in his.