The goal of this study is to develop a humanoid robot that can conduct a continuous conversation with people. Spoken dialogue systems have recently been rapidly developed, but the existing systems are insufficient for continuous use because they fail to inspire the user's motivation to talk with them. This is because a user is unable to feel that a robot possesses its own intentions, and thus the robot must acquire its own values to convey its sense of intentionality to users through its spoken communications. This paper focuses on a dialogue strategy aimed at building people's motivation under the assumption that the robot has a values-based dialogue system. People's motivation can be influenced by the intentionality as well as by the affinity of the robot. We hypothesized that there is a good disagreement/agreement ratio in a conversation to efficiently balance people's feelings of intentionality and affinity. The result of a psychological experiment using an android robot partially supported our hypothesis.