We treat dreams as evidence on a par with real events. From the latest Journal of Personality and Social Psychology:
This research investigated laypeople's interpretation of their dreams. Participants from both Eastern and Western cultures believed that dreams contain hidden truths (Study 1) and considered dreams to provide more meaningful information about the world than similar waking thoughts (Studies 2 and 3). The meaningfulness attributed to specific dreams, however, was moderated by the extent to which the content of those dreams accorded with participants' preexisting beliefs–from the theories they endorsed to attitudes toward acquaintances, relationships with friends, and faith in God (Studies 3-6). Finally, dream content influenced judgment: Participants reported greater affection for a friend after considering a dream in which a friend protected rather than betrayed them (Study 5) and were equally reluctant to fly after dreaming or learning of a plane crash (Studies 2 and 3). Together, these results suggest that people engage in motivated interpretation of their dreams and that these interpretations impact their everyday lives.
I'll bet we process dream experiences much like we process fictional experiences. Our tendency to treat fictional and dream experience as real evidence helps us to credibly believe things that it is in our interest for others to think we believe.