Never do these manuals stop to consider that dreams might be based on memories of daytime experiences. They always start from the assumption that all concrete objects are metaphors for suppressed emotions and anxieties, often of a sexual nature. Where they get these explanations I have no idea. It all sounds to me like a rather clichéd attempt to apply literary analysis (with Freudian overtones) to dream narratives.
Still, it's curious that some dreams seem so widespread. For example, lots of people report dreaming about accidentally going outside without clothing. Other common dreams include flying, or falling, or having one's teeth fall out, or facing an exam after having forgotten to come to class the entire semester. If dreams are particular to individuals, why are some of the most absurd ones nearly universal?
My personal theory is that dreams depict events that we expect might happen based on our life experiences. Many dreams consist of little more than distorted memories mixed together. Whenever I dream about an experience I've never had, I get the sense that my dreaming mind believes it could happen. There are things I desire which I never dream about, because I don't expect them to happen. Thus, I'm inclined to reject Freud's theory of dreams as wish fulfillment, even though I know he would argue that my subconscious mind suppresses my true desires.
An important mechanism in dreams is exaggeration. I once dreamt I was visiting a friend who in real life has one cat, but in the dream had five. I've also had dreams in which my own pets spoke English--not just my parrot, but my cats as well. I believe this reflects the way I anthropomorphize animals, thinking of them in such human terms that I practically expect them to talk. Only common sense (which I lack during sleep) makes me know better.
What about nightmares? How do they fit my theory? The answer is that when we're anxious about something, some part of us expects it to happen. That explains the feeling of inevitability in nightmares, nicely captured in The Wizard of Oz when the lion appears as soon as Dorothy begins worrying about "lions and tigers and bears." Those dreams in which we triumph over scary things, as Dorothy ultimately does, reflect our confidence.
As for the dreams where we walk outside without clothes, I suspect that they stem from our anxiety over relying on habit to get through daily routines. Most of the time when we dress ourselves, we aren't thinking about the activity at all. Just as we're occasionally forgetful with other habitual activities, it is semi-plausible (and I believe it has happened) that someone might forget to put on a vital piece of clothing. In the embellished world of a dream, that fear translates into reality.
As it stands, scientists now believe that REM sleep (the period in which most of our dreams occur) serves a memory-related function. It almost appears as if we're not meant to remember any of our dreams. We experience a kind of amnesia upon awakening, in which we quickly forget what we were dreaming about, unless we make a special effort to remember.
There are people who claim never to dream, but scientists assert that everybody dreams. Some people just don't remember. How do scientists know? In the sleep experiments of the 1950s which revolutionized the field, all the subjects, even those who initially denied ever dreaming, ended up reporting dreams at the moment the researchers awoke them from REM sleep.
The study of dreams presents many scientific difficulties. If a person says, "I dreamt I was a dog last night," we have to take his word for it. It's like basing our testimony on an amnesia patient who was the only witness to an event. We know that dreams don't occur in physical reality, but how can we verify that they occur at all? How do we know, as scientists currently believe, that they are hallucinatory experiences that occur in real time during REM sleep?
The 1950s researchers would awake sleepers who had just gone through a REM cycle (detected with an electroencephalogram) and ask them to approximate how long their dream had lasted. The scientists found a positive correlation between the length of the dream reported and the length of the REM cycle observed. They also studied the effects of external stimuli on dreams. In one case, a scientist dripped water down a subject's back thirty seconds before waking him. The subject reported having dreamt about singing at an opera, when molten wax suddenly began dripping from the ceiling.
The best book on dreaming I've read is J. Allan Hobson's 1988 The Dreaming Brain, which includes a remarkable amount of scientific information (much of which went over my head) as well as a theory that ties these observations together with stunning simplicity. The book is one of a kind: most books on dreaming focus either on the subjective aspects (and usually wind up sounding hokey and unscientific) or purely on the scientific observations (which are too limited to tell us much about the subjective state of dreaming). Hobson finds a middle ground where he is able to quantify the characteristics of dreaming and correlate them with the findings of neurobiology and sleep research.
I learned from this book that the portions of our brain governing movement and sight are activated during REM sleep. Hobson proposes that the ultimate source of dream sensation--which seems to be predominantly visual and motor--is physical rather than psychological. It is no wonder that dream sensations feel largely involuntary. As Hobson puts it, "we seldom have the experience of willing the movements that occur in dreams, but instead experience a sense of compelled or involuntary motor activity over which we have little or sometimes no control. This contrast is particularly and strikingly sharp in those dreams in which motor activity becomes a central part of the plot: for example, when one attempts to escape from a pursuer" (p. 171). This also helps explain the sensation of rapid movement we frequently experience during dreams. If we're anxious, we perceive that we're falling; otherwise, we may perceive that we're flying.
Since the sensations are physiological in origin, Freud was wrong: dreams are not messages from our subconscious (or from anywhere else, for that matter). But here's the catch: our thought reflections, which help organize the sensations into a narrative, infuse the dream with meaning. This meaning, according to Hobson, is usually transparent rather than oblique:
The activated brain-mind does its best to attribute meaning to the internally generated signals. It is this synthetic effort that gives our dreams their impressive thematic coherence: dream plots remain remarkably intact despite their orientational disorganization. And it may be that their symbolic, prophetic character arises from the integrative strain of this synthetic effort. The brain-mind may need to call upon its deepest myths to find a narrative frame that can contain the data. Hence, one can continue to interpret dreams metaphorically, and even in terms of the dynamically repressed unconscious, if one so chooses. But such a practice is no longer either necessary or sufficient as an explanation of either the origin or the nature of dreaming. (p. 214)A recent dream of mine exemplifies this discrepancy. I dreamt that people's faces kept turning into non-digital clocks. Interpreting this dream reveals the different possible ways of answering the question "Why did I have this dream?" It could have been that I had been walking around in the early morning and glanced at a non-digital clock on the wall; the image stayed in my mind so that when I conjured up images of people's faces, my memory of the clock came back to me. Dreams are good at finding analogies. But the dream may also have reflected my feelings over my grandfather's recent death, an event that has made me think lately about how life is finite. In the dream, I expressed this feeling by seeing people as ticking clocks.
Symbolic dreams of this kind are probably a function of the individual dreamer, rather than an expression of universal mythic archetypes. People who have symbolic dreams are probably symbolic thinkers; concrete individuals will tend to dream in concrete terms. Still, human dreams might have a lot to do with why we exhibit more creativity than other species. The biochemist Otto Loewi claimed that the experiment which led him to the postulations of chemical neutrotransmission--and earned him a Nobel Prize--occurred to him in a dream, and Robert Louis Stevenson based his story "Dr. Jekyll and Mr. Hyde" on a dream.
The important point is that our reactions to dream sensations prove just as important as the sensations themselves in establishing the dream "plot." This is not to suggest that dreams have no meaning. They certainly do, but according to Hobson, the meaning is usually more transparent than both Freudians and mystics would believe. Don't waste your money on a pseudoscientific dream manual; most likely, you are your own best dream manual.