Why people are unjust and mean sometimes?

Yesterday was the worst day of all, I guess. A woman came to pick up her prescription the day before yesterday but couldn't tell us her Medicaid number or any other numbers. We gently asked her to provide at least some information so that we could bill.

She, on the other hand, literally commanded us to call the doctor's office to get the information.

The same lady came yesterday, after a long phone call, demanding that her other prescription be transferred from Walgreens (I work at Walmart though), even though she didn't know which prescription it was!

People come at the last moment for antibiotics and other stuff, and when we tell them to sit on the bench because we will call them shortly, they just walk around and don't show up when the prescription is ready.

I mean, what's wrong with people? I work with a very nice team and under a nice and hardworking manager. But people's attitudes make the day very bad. Sometimes, it feels like they are abusing us.