My friend Sara, who has a Ph.D. in psychology, wrote in her blog today about dreaming's influence on inspiration, and incidentally why psychoanalysis isn't science:
REM (dream) sleep specifically is associated with increased abstract reasoning as well as increasing the strength of normally weak associations in the brain (see here). What that means is, two different things that your waking brain might not even see a connection between could become associated rather easily in a dream. Our brain does this kind of linking (neural networking) naturally, and it's a great way to learn new things - by connecting new knowledge you need to memorize to something you already know. Just as you can connect any actor to Kevin Bacon, you can connect any concept in your brain to another. Some connections are more direct than others.
Of course, Freud and other psychoanalytic theorists would state that these strange juxtapositions are simply your subconscious trying to work out any conflicts you're having in your life. In fact, pretty much everything goes back to this idea of the subconscious, at least for psychoanalysts.
Why do we need ... theory and hypothesis if Freud's and other psychoanalysts' theories can sum much of this up, in a neat, Oedipus-complex-themed package? After all, parsimony is an important aspect of science - the simplest explanation tends to be the best one, in the absence of evidence to support one over the other.
But that's the thing - the various social psychological theories outlined above have just that: evidence. Specifically empirical evidence, which is pretty important for science, something I've also blogged about before. In fact, psychoanalytic theories lack the basic ingredients that make them at all scientific: the ability to test these concepts (we call this 'testability') and, if they are false, demonstrate that (we call this 'falsifiability'). If there really are subconscious forces operating in your brain, trying to give you glimpses of what's really bothering you (latent content) but hiding behind symbolism (manifest content), how would we even begin to test this? After all, they're subconscious. But for social psychological theories, such as priming, we may know what evidence we would observe if priming happened and what we would observe if it isn't happening.
You can read Sara's other thoughts on psychology and horror movies at Deeply Trivial.