chloroformmmmm
What happens when a human falls in love with a skeleton?!
Sans is a cold skeleton. He's not open to new people, especially humans. One day, when he comes across a Human, on the verge of passing out from the cold, He decides to help her.
Something about her just makes him feel like there's something more to his life. He can't stop the blue hue dusting his face every time he thinks of her.
He is positive that She will never feel the same however. So he refuses to confess or even like the Human get somewhat close to him. He cannot risk losing her.