Same age,
Same School,
Same room. What else could've gotten worse when two guys have different feelings towards each other. Different attraction.

Why would Luhan hate his roommate SO MUCH without any logical reasons? 
Why would Luhan bully his poor innocent roommate? Sehun.

He never did anything bad towards Luhan. 
 

Maybe ever since Sehun confessed his feelings towards Luhan. Just 3 months being roommates, and Sehun confessed.
Isn't that so, bizzare.