The literature suggests that expressed gratitude improves the interpersonal relationship between a beneficiary and a benefactor. However, there is little research that has explored why thanking provides these positive effects, so this study investigated thanking mechanisms to explain reasons why people feel close to a beneficiary who expresses gratitude. This study also examines the effects of apologies, which are sometimes used to show gratitude in Japan. In this experimental study, 671 Japanese participants reported their perceived closeness, warmth, conscientiousness, and agreeableness to a hypothetical beneficiary who expressed gratitude, apologies, or both after a benefit was provided. The results revealed that benefactors who received a message indicating gratitude and both gratitude and apologies reported higher levels of closeness toward a beneficiary than those who received a message with only apologies and a message without either gratitude or apologies. A structural equation model further indicated that warmth and conscientiousness mediated the link between expressed gratitude/apologies and perceived closeness.