9. Hanging Onto His Spoilers
The Doctor had hundreds of adventures with River Song. It was perfectly ordinary for her to carve "hello sweetie" in ancient Gallifreyan and expect him to show up outside an airlock! For most of the episodes we saw, River knew more than the Doctor. That's for the best, really: she was always cooler when she was smugly lording it over him. But after a while, the tide turned and River stopped saying "spoilers." He started saying "spoilers." The Doctor knew all the things that were going to happen to River. He knew she was going to die at the library, and he knew he would send her to her death. He knew that, as long as he kept turning up in her life, he would dominate her life. He knew all these things, but he didn't act on any of his foreknowledge. Why didn't he? Why didn't he tell her "stay away from me," or "I'm not going to come when you call anymore: put together a life for yourself." Or "don't go to the library." He didn't because he already knew he hadn't, and he couldn't risk not doing what he already knew he did. He couldn't risk the paradox, so he didn't. And River lived and died a tragedy.