Why Do We Dream?

Nobody knows for certain why we dream. One of the first people to come close to a scientific explanation was the Greek philosopher Aristotle. At first, he spoke of the soul exercising special clairvoyant powers, in accord with its divine nature, when freed in sleep from the body's constraint. However, he was to conclude that the function of sleep and dreams was to dissipate the vapors that rose from the stomach after food. Similarly, for many centuries it was believed that blood rose to the brain and caused congestion there. Sleep enabled the blood to drain back into the rest of the body.

It is also from a psychological point of view, that dreams are a way of our subconscious working through our hopes, fears, ambitions and other such things. But we also only dream during the REM cycle of sleep which some people don't always reach and takes a minimum of 90 minutes or an hour and a half to reach.