Luhan's family was poor and needed shelter, so they were taken under the Oh's wing. Once Luhan had grown up, he fate had already been sealed, as he was chosen to be the Oh's son's servant. But it had it's perks. Luhan became Sehun's best and closest friend. 

When Kai playfully tries out an app on his phone that can give you an realistic image of your future child, the image that is displayed gives him the shock of his life.  His 'future child' has a striking resemblence to Do Kyungsoo, his best friend.