Duty has to do with moral obligation, what you ought or ought not to do. But obviously you’re not morally obligated to do something just because it would be good for you to do it. For example, it would be good for you to become a doctor, but you’re not morally obligated to become a doctor. After all, it would also be good for you to become a homemaker or a farmer or a diplomat, but you can’t do them all.
