Maybe it was because their fathers were old-time friends, or rather it was because their fathers' would constantly compare the two. Maybe it all had to do with their competetive personalities or rather both wanted to show the other up in some way. Maybe it was destiny for Oh Sehun and Kim Jongin to hate each other but maybe it was just a whole lot of perfectly-timed disasters.

Perhaps everything just traced back to the time Jongin snatched Sehun's stuffed bear when they were three and that is the reason for all of these misunderstandings now... 

Whatever it was it did not change the fact that Oh Sehun and Kim Jongin hated each other. They were constantly seen as SM High's Bestest Best Friends Forever but in reality neither of two could stand the other. And what's worst? Now the boys are given a reason to fight openly: new transfer student Do Kyungsoo.