You were supposed to share a dorm with Pandora Roiser. But somehow, your brother, Regulus, had convinced you to switch with him. You agreed, not knowing who his roommate was supposed to be. When you found out who it was, you understood why Reggie wanted to switch.

Severus Snape. The one who despised almost everyone with a passion. That was your new roommate? Great.

The door was locked when you came back to your dorm room after studying in the library with Barty Crouch Jr. You knocked on it and after a few seconds, you heard it unlock, and Snape opened the door.