Why should we be nice to rude people?

Well I work at a local grocery store in a rural Missouri town and almost all of the customers (maybe 85%) are just outright rude and immature. Keep in mind that these are adults and I'm just a 17 year old cashier. I am polite to them for the obvious reason of keeping my job (at least until I graduate this next May :D) but why should I other than that? They treat me like **** and I'm nothing but nice to them. I can handle it but it just irritates me. Why should we be nice to rude, immature people?