The Doctor watched sadly as two trails of tears formed on Amy's cheeks. She was crying for Rory, the fiancé she'd lost, but could not remember. He knew in a couple of seconds Amy would quickly brush the tears away in confusion, still wondering what made her so sad.
To be brutally honest the Doctor almost envied her. Never having to remember the pain of losing the one she loved. Amy never had to carry around the emptiness in her heart.
Sometimes the Doctor wanted to forget. It would be easier. He wanted to forget the desperate look on Rose's face as she fell into the Void. He wanted to forget the tears she had shed as she had said goodbye. He wanted to forget the feeling of helplessness and regret as he watched her kiss his other self.
But then he remembered her telling him she loved him, and he remembered the Bad Wolf (I want you safe, my Doctor), and the apple grass, and 'Run!', and 'it's better with two', and he decided the pain was worth it.
Maybe it was a way to show those who'd been lost that they were still loved and remembered, even after they'd gone.