Yeah, it's part of being human. Just like any other animal, we have drives to feel more superior than others. The problem with it, though, is that being human requires patience and it requires inhibition of some drives, such as superiority.
Yes, there is a reason for bullying, people do it because they feel like crap and try to make others feel the same way to feel less crappy. Some people also do it to show off to other people and try to get friends.
Possible indications of low self esteem. Maybe the subject bullying is dealing Experience of failure in school, sports, or other activities. Being ridiculed or teased. Physical, verbal and/or sexual abuse. Being highly criticized by parents, peers, or teachers. Being expected to be “perfect” at all times. Unrealistic expectations from parents, peers, and or/ teachers. Overprotective and/or authoritarian parents. Even though there is no excuse for bullying theres a foundation were it starts.
Honestly, they don't bully to make themselves feel better. I know this because I used to hang with the "cool" crowd in high school. I bullied people all the time. I still very much regret it. I did it to make myself look cool. That's the only reason most of the time.
If they're being bullied at a school, they should seek assistance from an adult such as a teacher, counsellor, principal or coach. There are avenues that your friend can pursue to put a stop to the bullying. No one should have to be harassed while they're trying to get an education.
Sometimes one has to stand up for themselves but if you're friend is not the fighting kind of person then any adult teacher or counselor will put a stop to it. Schools are fighting back now and they will NOT tolerate it any longer. If the bullying starts off school grounds then call the police an then tell the school of the incident. Students are protected from the time they enter school to the time they get home.