Why are people so unfriendly in supermarkets?

Because after a lot of time spent shopping, people can get very tired, worn down and agitated, I know my mum does but she keeps her cool and remains friendly at all times and she doesn't understand why others get so nasty to be honest, Nasty people make shopping a very unpleasent experience, if more people were friendly, more people would be happy, even if they are exhausted