Please some tell me why Indians hate Pakistanis?

I have been associated with US universities for about 10 years. As usual I come across lot of Indians and Chinese students as we all do. No matter where Indians are from southern, eastern, western or northern region. With lot of hatred between them they have one common enemy and that is Pakistanis. They are very nice at first and charming and sensible, but as soon as they find out that I am from Pakistan. They just started to behave like you know what? I am not talking about ignorant or illiterate people. I am talking about girls and boys who are pursing Graduate and Post Graduate studies. Is there some kind of brain washing going in India against Pakistanis. May be crooked politician see no other way to keep hundreds of Indians races to keep united unless they have one common enemy? I love India it’s a land of my parents and some of my distant relatives still live there. Half of women relative through marriage have come from India. I do not forget that in 1947 through very bloody senseless partition created Pakistan and India. I don’t want to go into this politics that India should or shouldn’t be divided. Please someone tell me in black and white why the kids coming to this world after forty and fifty years of partition hate us.