Why Do We Dream?
For the long answer, click here.
Basically, there is not strong agreement amongst scientists for "why do we dream?" When hypotheses have been tested, the null hypothesis usually comes out triumphant. Dreaming is a subjective phenomenon, and the experience of dreaming may serve some useful function for the animal having them (including humans), or dreaming could be an epiphenomenon which results from something underlying which is important (maybe REM sleep), while the dreaming itself is not important. This latter possibility could be the case because evolution does not eliminate mutations if they do not have a negative affect on an organism's survival. This is unlikely to be the case though because dreaming surely has an affect on our behaviors - whether subtle or pronounced, dreams often influence how we react to life the following day. Therefor, if something as ubiquitous as dreaming has any affect on the organism and species having them, that affect must be adaptive (historically speaking, maybe not in the current human environment). So, dreaming is probably an evolutionary adaptation. Hard to show experimentally how dreaming is or is not adaptive to an individual (ethical and study-protocol issues), but we can say confidently what we dream about and then attempt to extrapolate out the why and what.
Dreams are predominantly stories which deal with the "F's." Just so happens that fight or flight sympathetic nervous system activity is over-represented in dreams. Fighting and fleeing, but also fornication. Not so much feeding/food, but these creep in on occasion also. Doesn't stop there. Along with fighting, fleeing, fooling around, we also see a lot of falling, some flying, failures of all sorts and lots of (mis)fortunes. Sometimes we might find money in dreams or have some good fortune sprinkled in, but usually (for EVERYONE) dreams depict us navigating through situations where things are going wrong. We have a stronger than normal perseverance in dreams (necessary when we are trying to escape a monster and we feel like we're running in molasses), and the overall formula looks like DREAMING = THREATS + PERSEVERANCE
This all leads to the rational conclusion that dreaming is, at least in part, a threat simulating virtual environment that should prepare us to survive in our ancestral environment. Are there other evolutionarily adapted-for dimensions to dreaming? Maybe, but strong theories have not been presented for additional functions.
This is not to say that dreaming can not feel beautiful, creative, and happy. Or even mundane. Depends on your daily life, how your unique biology responds to stresses or lack thereof, and, if you "work" with your dreams, what you are doing with them. Regardless of what dreaming is for, the system can be hacked through lucid dream induction techniques and adventures impossible to achieve while awake can be realized. However, many lucid dreamers (without the above knowledge) are likely surprised at how dark and weird the content and themes of their lucid dreams actually is.