Everyone has different ideas about dreams but in real they themselves are not sure about it. One of the theories commonly said by doctors is that when u are at any place, say office home etc. A lot many events are going around and you neither think nor pay any attention towards them but when you are asleep you think about those events that are called dreams and of course how you feel for a person, thing or situation you recall all that during your sleep which causes dreams.
Some people think that pizza, burger, chocolate, soft drinks are responsible for their dreams but in actual they themselves not know much about it. Moreover it has not been proved yet.
In the beginning of 20th century SIGMUND FREUD gave his theory that our unfulfilled desires come in front of us in the form of dreams.
Another theory about it is that dreams are caused due to unexpressed feelings. People in depression dreams more than a normal individual.
Another saying is that subconscious mind makes the dream a vehicle to let your conscious mind aware
about what you think and in what way. Or you can say that your own internal voice talks to you during sleep and it knows about you better than you do.
Some people believe that the dreams are the work of aliens or demons who wants to convey us some message and others totally denies this theory.
It is said that dreaming is necessary for healthy, and happy life.