Luhan and Sehun were in love and everyone knew it, even management. 

But since Xiumin became so popular, they wanted to pair him up with Luhan.  

They made Hunhan break up and Xiuhan the new "it couple"

Eventually they really caught feelings and fell in love, but Luhan still loved Sehun. 

So he snuck around behind Xiumin's back with Sehun, not taking his feelings into consideration. 

Though he got caught, he did it again. Sehun wasn't completely against it, because he still loved Luhan, but he was worried about Xiumin. 

But what goes around comes around , right?