Understanding dreams involves understanding ancient history to some extent.
In Old Testament times, dreams were a means of getting information from the spirit world to our own world, thus Daniel and Moses were able to interpret dreams and make sense of them.
There were also a number of arcane practices meant to facilitate contact with the spirit realm a bit more directly: prophets, the Greek oracles, witchcraft/familiar-spirits (the story about Saul, Samuel, and the 'witch of Endor' for instance), and also pure electrostatic devices like the ark of the covenant.
All of those things involved trance states like hypnosis, all involved static electricity, and all of them stopped working before the time of Alexander.
Likewise dreams no longer serve any useful purpose and are basically like listening to static on a radio; you're just picking up bits and pieces of stuff but never enough to really tell you anything. Our ability to dream is what remains of a function which we no longer really have, sort of like an ostrich's wings.