I think you just go to the wrong stores. I go to a lot of stores, both close to my house and in other parts of the city, where argentine workers are friendly and nice to me. In my neighborhood, I have known many of them for up to 10 years. That helps, of course.
I also often try to communicate, making the humanity a two way street.
Also, I avoid Coto or Jumbo or similar big chains- I go to small, family owned businesses, both here and in the USA, whenever possible. I buy my granola at one dietetica, for example, a few blocks away, run by a very nice older couple, and for years, they have always been friendly to every customer who walks in. Versus New Garden, with many locations around the city, and lots of bored 20 somethings working there who would rather be somewhere else. My local pasta makers, greengrocers, bakeries, and even chinos are all pretty friendly. One of the greengrocers even gave my wife a christmas present last year- a keychain with flashlight. A small thing, but certainly more than you would ever get by saving a peso at Carrefour.