Is it because they want to tell other people that their child topped in class? I mean, why are their expectations high? Why aren't they ever satisfied with anything below 90%?
Parents live through their children. It is sad but true. Parents also make a lot of sacrifices for their children, so they want them to pay off. Finally, parents often want their children to have a better life than they did, so they have big goals for them.
I feel that the biggest fear of all parents is to see their children's lives wasted away. It is no different than losing a child to a slow disease, and should not feel any less desperate than seeing a kid wasting away on drugs. When one sees a perfectly healthy and capable child not trying at all to fulfill their potential (and this, I am saying wearing my "teacher's hat", not my "mother's hat") the feeling can become overwhelming; you just have to ask yourself: if you do not see how much you can give, what in the world could I ever do to help you see it!?
However, I do agree with the previous post in that there is another side to this; that there is also the possibility that some parents were THE ONES who wasted their lives away and now pretend to push onto their children all the discipline to succeed in what they (the parents) couldn't. This is because their kids, they feel, are reflections of them. What parent who spent his entire teenage years smoking weed and drinking, dropping out of school, or acting a fool would want their children to reflect that same behavior? I am sure some parents are dysfunctional enough not to care, but a normal parent may want to revert all that and see his child succeed.
Often parents relive their own youths vicariously through their children. So, if their children do well and are successful academically or athletically, there is a sense of redemption in the parents for their own failures to measure up to what they wanted to be. In addition, since children are a reflection of their parents, there is a sense of pride that parents can enjoy when their children do well. Finally, if their children achieve much, there is a greater sense of worth that parents may feel as they reason that they must be worthy if their progeny can do so well. But, more than anything, if parents love their children, they want them to reach their full potential so that they will not have the feelings of disappointment in themselves and will, instead, live satisfying and happy lives.
Parents are automatically wired to want their kids to do well. It's part of the parents nurturing reflex. It can cause conflicts when kids don't agree with what parents think is for their good.
Sometimes parents are also concerned about how their kids' behavior reflects on them as parents. This is perhaps a more self-centered reason for wanting your kids to do well.
But for the most part, I believe it's a God-given impulse that works as part of the natural bond between parents and their children.
This is surely different for different parents. However, I think there can be two main reasons. First, they want their child to fulfill their potential. If you know your child CAN be the best, you want them to actually BE the best so as not to waste their talent. Second, they want a good future for their child. They want the child to be able to go to a good college and get a good job. This can drive parents to be demanding.