What makes people think it's ok to be so rude?

I’m really curious as to what makes people think it’s ok to commit random acts of rudeness.
I work in retail and it’s just unbelievable to me the way that some people act. Are they raised that way? Are they so self absorbed that they just don’t give a shit about anybody other than themselves?
Whatever it is, I find it disgusting and I’m finding it harder and harder to keep myself in check.
For example, the other day I had a lady come in and she was asking for the impossible. I tried to explain this too her and she told me that I “must be stupid”. Then proceeded to toss a bunch of papers at me. I tossed them right back at her and said “actually, I’m not stupid and you’re very rude! If you don’t like the way that I do things, please feel free to leave”. But oh no, that would be too easy, she’d rather stay and piss me off even further. I finally just called my manager over before I slapped her and got myself fired.

I would never even dream of treating a perfect stranger that way. It baffles me that so many people these days have such a lack of manners.