Why do we do what we do?

I just got a phone call to set up an interview, which will be the third interview I've had in one week (all different schools/companies). While researching daycare options for my children and realizing that even if I work full time as a substitute teacher, my take home pay will be minimum and I will lose out on the time I treasure with my small children. As a substitute you only get paid per day and for the number of days you work. Daycares, however, require you to pay per week regardless of how many days your children are physically there. So in essence, I could be paying more than I actually make.
Then I read an article published by Science Mag titled "Human Hearts Beat Together" which made me realize that a baby's mother is absolutely irreplaceable by any other care giver in the world, no matter how talented s/he is. I also recently read a different article citing that currently only 20% of families are considered "traditional" in that the mother (or one parent) stays home with the children. Meaning that the other 80% of families have dual working parents.
It's got me thinking. Why do we do what we do?
Why go back to work? Why stay home with the kids? Is it purely financial?
So, please tell me, why do you work? Do you enjoy what you do? Why do you stay home? Do you find your role(s) fulfilling? If you were to find out that your time left on earth is drastically limited, would you be happy with the choices you've made or are making?
Please comment either within the blog or on facebook.
THANKS!