Service industry people have to deal with customers. Even in an age of robotic or outsourced (Indian) phone labour, there's still people whose job means face to face contact.
Contact means conversation. And, since they're serving you, it means they have to laugh at your dumb arsed lame attempts at humour.
It's like a rule. I've never gone through customer service training, but I bet that rule is on a powerpoint slide somewhere in the induction package.
I had my hair cut recently. It had been a while. And I am one of those people who try to spew forth semi-amusing commentary about, well, anything.
In this case it was topical - my hair. I have a bald patch and back hair. It's a horrid combo.
My (witty) observation?
'My hair's migrating south. By the time I'm ninety it will puddle around my feet like a Hobbit'
Cue obligatory laugh from attractive short (but well styled) chemical blonde haired hairdresser.
I don't blame her for (presumed) fake laughing. I know she has to do it.
Otherwise ... well ... cough cough tumbleweed...