Inferiority. Believe it or not, bullies tend to have inferiority complexes, often from having been bullied themselves. and yes, it can continue into adult life. Tell someone you know and trust about the bullying. Keeping it inside does no good.
Report the bully. Bullying goes back to survival of the fittest from evolution. Bullies act like Alpha males who want to impress others including females to make themselves more desire able. You see the same in the wild with other animals besides humans who exhibit the same aggressive behavior. Bullying is not tolerated anymore. If you are in school file a report to the school guidance counselor or resource office.