You're his enemy, you don't wanna be his enemy, but you're also friends with Genya *his brother* so that's why he doesn't talk to you. If you watch Demon Slayer then you know what happened. Anyway, you were having a mental breakdown in your room and Sanemi could hear sobbing coming from your room. He then looks through the small crack from your door, he saw you curled up on the floor. He then knew what to do. So he opened the door and checked on you.