It's could that people don't realize how much this job asks of you until you get into it yourself. I feel the same. I had most time to myself when I was single, it reduced to half when I became double, reduced more than half again when I became triple.

Even than, the joy you get with your own kid can not be compared with anything you ever had before. The job is very tiring yet very fulfilling. Don't think it can be explained until you experience yourself.