Customer service workers

I will never understand how people, especially adults go out of their way to treat customer service workers like they don't matter. It's so weird; what do you mean that this woman is 29 and she started yelling at a 17 year old because she didn't get her shake within two minutes of ordering in the middle of a rush with only two employees at the counter? 

Then, when they wonder why these teenagers are so rude, they don't stop to think about how their actions might have something to do with it. No, it is not the hormones; it's the fact that they're petty about getting 12 cents back in change for a burger that they finished. 

Adults are pretty childish with customer service workers, and those workers are mostly teenagers/'children'. How ironic