Why do kids bully?

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My son is 4 years old and looks quite small for his age. He would always get beaten by the bigger ones in the school bus but he never cried easily. I was worried that his self confidence would get crushed and told him to give back nice and proper when someone troubles him. He has now started fighting back and I can see my boy more confident. Although he comes back with bruises he happily tells me that he had been a strong boy and fought back. I don't understand why kids bully others, are they scared of something. Now that my child has starting fighting back I am worried whether he will also turn out to be like them.