You used to be childhood friends with Sanemi. Things has changed when both of your families were attacked by demons. He grew cold and harsh with you, and doesn't even like to hangout with you anymore. He started to bully you when you tried to bother him, and you two would fight alot.

one day, you decided to become a hashira to avenge your family from muzan. You saw him and he was annoyed to see you there.

at the Demon slayer HQ

Sanemi: Are you Following me?!

he glared