I think lots of factors could be at play here
Being a parent can be a great joy, unless Billy isn't such a great kid. Then it becomes a burden. You have goals and aspiration for your children and they come along and poop all over your dreams. Then outside of that, you has a provider during a down economy, have the constant stress and worry that you might not be able to provide.
Now if you have the prefect kids and they pursue the perfect plan you had for them and job worries are never a concern, then you're happiness will be through the roof.