This paper investigated the influence of a social robot which discloses a backstory of its experiences on the development of trust in human-robot interaction with respect to the nature of backstories. We compared three cases of backstories, a happy backstory, a sorrowful backstory and no backstory told by the robot during interaction with participants. The results indicated that the robot disclosing a happy backstory provided the participants with higher impression of trustworthiness in general and affective trust compared to the robot telling no backstory. However, the robot with sorrowful backstory was not evaluated to lead to higher trustworthiness than the robot with no backstory. Furthermore, the happy backstory condition scored higher than the sorrowful backstory condition in general, affective and cognitive trust. Thus, participants rated a happy backstory tied to positive self-disclosed emotion, to be significantly more influential in human-robot trust.