Why do parents demand so much from their teenagers?

Grown-ups love figures. When you tell them that you have made a new friend, they never ask you any questions about essential matters. They never say to you, “What does his voice sound like? What games does he love best? Does he collect butterflies?” Instead, they demand: “How old is he? How many brothers has he? How much does he weigh? How much money does his father make?” Only from these figures do they think they have learned anything about him. ~Antoine de Saint-Exupéry, The Little Prince, 1943, translated from French.

Why Would Parents Demand Much From Their Children?
Here are several factors:
Family background
Continuing the family ‘legacy’ is important for most parents. For instance, if you come from a family of doctors, chances are, you will be expected to become one, too, regardless of your capacity or inclination. When one or both parents are achievers, they don’t see any reason for their talents not to manifest themselves in their children.
Economic stress
Some average income earners force kids to excel beyond their abilities so they can avail of scholarship grants and minimize the cost of schooling in their budget.
Children owe it to them
Many parents think that the formula to their children’s success is to provide them with everything. And since everything is given, there is no reason why they cannot excel.
“If her child can do it, why can’t mine?” Hearing parents boast about their kids’ success causes other parents to fell envy and even self-doubt. Because they feel this way, parents then put more pressure on their kid.