It was supposed to be a romantic dinner, but the silence was of the thick and uncomfortable variety. The pair of scientists sat across from each other, idly spooning at their soup.
“I’m not getting any younger, you know,” she said.
“Yeah, I know,” he replied glumly, “Neither am I.” The brief conversation relapsed into silence, with only the clinking of spoons in bowls as the soup continued its inevitable descent to tepidity. He stopped, held his spoon perfectly still, and said with dawning realization, “We really should do something about it.”
So they did. They built a time machine so they could go back and be younger, and it worked perfectly.
Now most people would expect that this story ends with them living happily after whenever. But what really happened was that their relationship was over before it had even begun.