Why parents push their children to fulfill their unfulfiled dreams & kill the dreams of childrens?

it is a very difficult question...it is very easy to say that each child should full fill his dreams but how it is possible...some time children want to be an actor or pilot or sportsman or politician or doctor or engineer or journalist....but to fullfill a dream one doesnt need only talent but also some luck and finance...so parent and children should have a joint discussion on this topic...both should know each others capability as well as capacity.