Actually, 'should' is an absolute. So while we prefer to speak of taking care of children or doing 'as we agreed to', we know that those are not things that we would do absolutely. Thus it is more accurate and healthier to describe our belief more clearly.
A simple approach to the belief is to ask 'why?' or 'what if?'--recognizing that we are using an extreme example in order to make the point.
So 'we should take care of our children'--why? what if someone else can take better care of them? what if their care is beyond our capabilities?
The point is that there are things we value highly, beliefs we hold dear, but if they are absolutes they can trap us into behaviors that are unhealthy for us and for the other. People in "co-dependent" relationships probably know that better than anyone.