Unlike prior research related to salespeople, this study focuses on the positive affective state of salespeople as a factor influencing their selling behavior. This focus is justified because the mutual relationship between customers and salespeople is crucial for customers to experience positive emotions during their interactions with salespeople, and the affective state of salespeople during the service delivery process is extremely important for facilitating such mutual relationships. This study proposes happiness as a critical affective state that can induce positive behavior in salespeople. Happiness is a relatively enduring and stable psychological state and a positive affective state that does not change easily. Therefore, it is a factor that can elicit relatively sustained positive behavior during the customer interaction process, and consequently, happy salespeople are likely to show considerable differences compared to unhappy salespeople in terms of customer consideration and individual performance from a long-term perspective. Accordingly, this study applies individual happiness to the sales context and focuses on the factors influencing salesperson happiness and the selling behaviors resulting from such happiness. The research results revealed that personality traits (extraversion, neuroticism, self-esteem) and social relationships (supervisor, coworker, family) influence happiness, and that this happiness in turn affects salespeople's customer-oriented selling behavior and positive emotional expression. Furthermore, salespeople's customer-oriented selling behavior and positive emotional expression were found to influence customers' perceived service quality. This study proposes employee happiness—a positive affective state—as a factor that can increase salespeople's positive selling behavior and thereby enhance customers' perceived service quality, and provides strategic implications regarding the importance of salesperson management that can offer psychological and mental satisfaction beyond material rewards as a means to promote positive selling behavior.