River immediately knew something was up. Her first sign was his pensiveness. He was always pensive, that was just his nature, but this was a different pensive; it was deeper, sadder, but not entirely sad. He'd been like that all day. Now he was standing in the control room, subconsciously messing with different levers and knobs. She wasn't even sure if he realized she was still in the room. She wanted to do something to get him back to his normal self. Figuring out what the problem could be was a good place to start.
She walked up behind her husband and put her arms around him from behind, giving him her normal flirtatious smile. "What are you doing?"
"I…" He looked down at the Tardis console and seemed to realize for the first time where he was. "I seem to be messing with the console."
"Well what are you thinking about?" she asked, kissing him on the cheek.
He gave a weary sigh. "Just stuff."
"Tell me."
The Doctor opened his mouth as if to speak, closed it, then opened it again. "Why does… why does time keep going?"
So that was the cause of his being pensive. "Well… You know why." She paused, thinking. "It goes on because it must."
"But why?"
"Because we do."
"We have to, we don't have a choice. Time doesn't wait for anyone to catch up."
"Maybe it's best that way. It keeps going because that's what it was always meant to do. Besides, if it didn't continue on the way it does, we would have no more good memories to make, no more stories to tell. Then there would be no point any longer."
He nodded silently, but she knew that he saw reason and truth in her words. River smiled. "Now, take me on an adventure. Let's show time that we'll use it properly. That it goes on."
He returned her loving smiled and moved the Tardis to do just that: take them on an adventure.