Why do people always like to call me big brother?

Hehe, I call you big brother just to bully you. Just kidding. I think they call you big brother because you are more approachable. As for their selfishness, this is just your opinion of them, and maybe they don't realize it themselves. It is also possible that they think that people with affinity are easier to bully, so they bully you. Sometimes people have to show their strong side, at least let others know that you are not so easy to bully. O(∩_∩)O~