Zoe and Dan didn't need anyone to tell them. They just knew.
It was just the order in which their lives had to come.
Saving the world comes before anything else.
Even when you're on your very first date, and you both laugh too much, because it seems too...normal to fit in with their lives.
Even when you're depressed and desperate, and wish that your life was that normal.
Even when you think your life may actually be in it's final moments.
The world still comes first.