In some cultures, introverts are esteemed over extroverts (e.g. China) but in the American culture, extroverts reign supreme. We thinks extroverts are more enjoyable to be around than introverts -- funnier, livelier, and more engaging. Americans often deny their more introverted selves, trying too hard to seem confident and happy (e.g. drinking too much alcohol at college/office parties).
Some American parents do not accept their introverted children and want to change them. They believe their children will have more success in life -- at school, in the work force -- if they're extroverts. Fortunately, because of places like Silicon Valley where introverts rule, parents are seeing that extroverts are not the only ones who can make it big.
For parents who are unaccepting of an introverted child, I highly recommend the book "Quiet: The Power of Introverts in a World That Can't Stop Talking" by Susan Cain. As an introvert myself, I found it extremely helpful and affirming.