Why are most people happier when their jobs are not related to their passions?

When a person's job is not related to his passion, he is free to pursue that passion unpaid, on his own terms, when he is not at his job. In addition, the number of jobs available for consideration is much larger, when not limited by passion. A person comes to realize that his work is valuable, whether paid or unpaid, and that even tasks such as parenting or cleaning house are just as important as paid employment.