An angry or upset customer is likely to feel frustrated because they are not satisfied with the product or service they received. Frustration often arises from a sense of disappointment, dissatisfaction, or unmet expectations. It can manifest as anger, annoyance, or irritation, and customers may express their frustration through complaints, negative feedback, or aggressive behavior. Frustration is a common emotion in customer service situations, and addressing it effectively is crucial for resolving the customer's concerns and ensuring their satisfaction.