He doesnt like endings.
The Ponds were what gave the Doctors life meaning. All three of them were out there, safe and happy, while he gallivanted through space and time. All three of them would be there when he came for them. Thats why the Doctor continued to travel with the Ponds, and refused to admit that, if they didnt stop traveling, either they or he would lose the other. The inevitability of death was something the Doctor used to understand, and sometimes even accept. But the Elevenths attachment to his Ponds made him resist the basic fact that an end was coming. Even at the very end of Amelias time with him, he missed his chance to say goodbye. River said goodbye to Amy, but the Doctor just kept begging her to stay with him.
The Doctor forgot how to accept that those he loved would die. And when he wasnt allowed to hide from that fact anymore, he refused to visit the slowly decaying River in her library afterlife and hid away on a cloud in Victorian London, removed from real life. He preferred isolation to the possibility of pains return. When losing the meaning in life hurt so badly, he wasnt willing to reach for more meaning. Which, as Clara pointed out, is a very human reaction.