1. A conversation-sentence generation device that generates a conversation sentence of a virtual agent having a personified conversation with a user, comprising: an input unit that receives, as input information, a conversation sentence given from the user to the agent, first clue information based on which a physical and psychological state of the agent is estimated, and second clue information based on which a physical and psychological state of the user is estimated; an agent state storing unit that stores the physical and psychological state of the agent as an agent state; a user state storing unit that stores the physical and psychological state of the user as a user state; an agent state estimating unit that estimates a new agent state based on the input information and the agent state; a user state estimating unit that estimates a new user state based on the input information and the user state; an utterance intention generating unit that generates, based on the input information, the agent state, and the user state, an utterance intention directed from the agent to the user; a conversation sentence generating unit that generates, based on the input information, the agent state, the user state, and the utterance intention, a conversation sentence given from the agent to the user; and an output unit that outputs the conversation sentence generated by the conversation sentence generating unit.