It's often because their expectations aren't met. For example, in restaurants, customers anticipate polite service, so if servers are impolite, it disappoints them. This applies to other industries too, with the belief that "The customer is always right." Many expect their wishes to be fulfilled without exceptions. When staff can't meet these high expectations, it can lead to frustration. However, customers shouldn't always expect perfection because sometimes their expectations are too high, and workers have limitations.

People often get dissatisfied with customer service because of staff attitude. If staff seem indifferent or don't show empathy, customers feel neglected. Also, if the staff appear uninterested or disconnected, it gives a bad impression, making customers unhappy.

I think people's unhappiness with customer service is often because of outside reasons. For instance, bad reviews on the internet or things people say on social media can make us expect less from a service. Also, what is good service in one place might not be the same somewhere else because of different cultures. This makes some people unhappy, especially if they're used to a certain way of being served.