Sans had always loved Frisk. Ever since she fell down into the Underground as a child, he was obsessed with her. Whenever she was hurt, he would try to save her, but just be stopped by his brother, Papyrus. He knew no one would accept his relationship with the human. Even when they got to the surface, the monsters didn't want Frisk dating anyone. So why did he get her pregnant when she was only sixteen? Was he marking her as his? Was he showing their true love? Whatever it was, both of them knew this: They'd have to run.