Many people want to HAVE children, but do not want to BE parents. HAVING children is like having a home or a job: children become accessories to our lives, picture-perfect cuties who are trotted out for everyone's admiration. When people HAVE children, they are embarrassed when the children misbehave, don't get good grades, or are not good at sports. People who HAVE children view their children as a reflection of themselves and their perfection or imperfection.
BEING a parent requires you turn your entire life inside and out. Your first thought is of your child, not yourself or how they make you look. BEING a parent means you look to your child's interest before your own; you allow a child to pursue his interests, even if you don't care for them yourself. It means your focus is always on the long term goal, not the short term reward. BEING a parent requires the kind of unselfishness that can only come in living FOR someone else.
Come to think of it, the same can be said for HAVING a spouse vs. BEING a spouse.