Why do Texans hate "yankees" so much?

Their hatred towards "yankees" seems really like they're ashamed of something. Like I mean what trash dumpster did they climb out of? Where do they get that everyone from the states north of them is considered a yankee? A "yankee" is just a different person than them......so why don't they like people who are different than them? I really don't understand what is so great about the south anyway the east coast knows how to do formality way better than the south in my opinion anyway. From what I see people in Texas are dumb as a box of rocks. & TX's colleges are not that great, people in Texas act like the colleges are elite but the colleges on the east coast mainly are where the elite colleges are at. The colleges in TX are equivalent to any other average college in the country.