Parents expect way too much from their kids.
Overly simple age-targeting is one main culprit. We all know that children develop differently, but it's natural to underestimate the astonishing variability among and within individuals. A child may be the first in her class to ride a two-wheeler but the last to learn to read; she may also grasp addition and subtraction well ahead of others but lag behind in achieving the self-control to short-circuit a tantrum. We also tend to parent subjectively, setting the behavior bar with a too-small sample group drawn from personal experience: our own first child, a neighbor's child, or our own unreliable childhood memories of how our parents raised us. (If you do want to compare a child constructively with others of the same age, the University of Michigan Medical Center's Web site offers a useful listing of developmental milestones.)
Our expectations of our children's psychological abilities, even more than of their physical abilities, are typically much too high. The research shows that we consistently overestimate their self-control, ability to persevere and stay on task, consistency of performance, and social ability. It's normal for a 2-year-old to get bent out of shape if he doesn't get something he wants; it's normal for a 3-year-old to lose it if there's an unexpected change in the bedtime routine; it's normal for a 6-year-old to fail to sustain focus on a baseball game, to pursue one fly ball with steely purpose and to let the next fall untouched in the grass because he's daydreaming. We know this, and we know that each of these developmental stages will probably pass in a few months' time, but, still, we stand over the child with index finger raised, an unpleasant edge in our voice, futilely repeating: "I said you'd get it later," or "Why are you making such a big deal about your bedtime story?" or "Get your head in the game!"
Necessity feeds this habit, and so does the human tendency to see the world according to personal priorities. If your work schedule obliges you to put your 3-year-old in preschool for 10 hours a day, you'll expect her to function peacefully there whether or not she's capable of it, and your own sense of sacrificing for the good of the family will encourage you to regard that expectation as reasonable. "I work and slave all day for your benefit, and all you have to do is play nicely with the other kids. So stop hitting them, or I'll have to spank you."
Frequently, we want something very simple from kids, like peace and quiet. Is that too much to ask for? Sometimes, it is. Come nap time, you may be thinking, "OK, I fed you, I changed you, I tucked you into your crib with your special blanket and teddy bear, I even bought this expensive mobile to hang over you. You're not teething—I checked. Everything's perfect. Children your age are supposed to take a nap. Your nap is scheduled for right now, and I have a phone call to make in nine minutes. Go to sleep right now!" If your child could articulate what's happening to him, he might respond, "I love the mobile, but my bones are growing like bamboo at the moment, and it hurts. I think I'll stay up and cry instead."