They were once pure, kind-hearted teens who loved everyone around them. But things change and so do people. They used to be tooo kind, that in the end, caused them to be taken advantage of. And thats why they changed. They vowed to become stronger and meaner thinking it would protect them from getting hurt again. 

 I recently found another story which has the similar storyline of this one where Wooyoung and IU both change due to something in their past. so before anyone frames me of plagerizing, im telling you that everything was my idea. and if there are other stories out there which are the same or similar, its just a coincidence.