Tim's realization that all the time-traveling in the world won't make Charlotte fall for him.
After his father dies of cancer, Tim travels back to see him, who tells him that he knew about his cancer, but could not undo his smoking, as doing so would have meant losing his children.
Tim time-traveling with his dad one last time before he has to let him go. It's more on the heartwarming side of tear-jerking.
