Bullying is mainly a way of gaining status and popularity for youth. They gain advantages for themselves by abusing those weaker than them. Think a bit about how offensive your question is, though. I am quite disgusted when people ask it from victims of bullying, it's not beneficial and shows a lack of empathy and understanding.

Imagine asking this about the Holocaust.

"I seriously cannot believe this if there is not a single reason WHY they did such horrible things"

Imagine asking this about slavery.

"I seriously cannot believe this if there is not a single reason WHY they did such horrible things"

Imagine asking that from a little girl who is tortured psychologically every day for years.