Though no one can quite recall who built it or why, it's the invisible fence we all consent to obey. The usual is what people do, that nebulous, non-elected committee that somehow decides how we're meant to dress, talk, love, work, and live. It is strong since it is unwritten, unstructured, imposed by nothing but arched eyebrows and whispered opinions.
The standard dictates that you go to school, get a job, get married, buy a house, have children, retire, perish. In that order if possible. Variance calls for justification. Why are you not yet married? When should you start having children? What are you doing with your life? The norm doesn't pose these questions; rather, other people do and they are channeling the norm, speaking on behalf of this shared expectation that none of us personally produced but all of us somehow uphold.