Why do we have so many wishes and desires?
In the context of evolution, desires and dreams (beyond the lower tier of Maslow's Hierarchy) could be seen as, not an adaptation, but a spandrel of the adaptation of consciousness.  

Consciousness allows us higher cognitive skills, particularly the ability to reflect on events and predict events through modelling, which allows us to modify our behavior in a succesful way.   But this extra predictive power also gives us an imagination with which we can imagine ourselves in a better position to exploit our dopamine centers (seek pleasurable activities that don't propagate the species).