Chances Lost (set during Donna's wedding reception in Runaway Bride)
As he stared at the dance floor at Donna's wedding the Doctor couldn't help but be filled with regret. It seemed as if the universe was mocking him.
He had just lost Rose without even having the time to say just three words. It was ironic, the Time Lord who ran out of Time.
And now, here he was, at a wedding reception watching people laugh and dance and celebrate two people's love for each other as he mourned the loss of his own love. It just served as a reminder of all the chances he had lost, all the things he would never get to do, or show her, and all he'd never said, or more specifically, three words he never said. I love you.