maybe the cashier doesn't like the OP's body language or the fact that they laugh while she is working.
OP's body language = genuine friendly smile coupled with 'hello'.
OP only started laughing after about 5 instances of real rudeness from this person. We are laughing because it's ridiculous and we can't help it. As I said, we switched cashiers because we didn't want to laugh at her.
This girl throws her eyes up to heaven. Never says please or thank you. Never smiles. Once an item didn't have a barcode on it so she said to me 'go and get a different one of these'. She never hands you your change; just leaves it on the counter even when you have your hand out - but she wouldn't know because she doesn't look at you.
I have genuinely considered that maybe she's depressed and therefore hesitate to even say anything to her. It's worth noting too that most of the customers in this supermarket are actually very nice because it is in a long established community. I would say the problem is just attitude and bad/no training at management level.