If you are concerned about something or worried about a future event, you might see it in a dream. The subconscious mind can send you your worries and concerns in the form of a dream.
If a person wants something badly, he might see himself achieving it in the dream. The dream in such a case can be a way the mind uses to satisfy the person's important needs (See also interpreting your dreams).
Some people consider dreams the thoughts of the subconscious mind. While the conscious mind thinks using logic, the subconscious mind thinks using symbols. Because the subconscious mind remains active during the night dreams can result from it's activity.
Some people believe that some dreams hold visions of the future. The vision can be direct or clear or it can also be encoded in the form of symbols.
Dreams can be a way the mind uses to motivate a person to achieve an important goal. If an exam is near then a person might dream that he is not prepared. By doing so the mind ensures that the person will feel like studying when he wakes up (See also why we dream about people).
One theory states that dreams are needed for survival. The theory assumes that dreams are used as a training ground for people to practice important survival skills that they might need in real life.
One theory states that dreams are the mind's way of storing recent events in the long term memory. Because events are stored in the form of symbols they are seen as dreams before they are placed in the long term memory (See why we dream in symbols).
One theory states that dreams has the purpose of cleaning the mind from unwanted thoughts. By arranging important thoughts and discarding unimportant ones, a person's mind can free a space for more important thoughts.
Some dreams can provide solutions to existing problems or hints that can help solving them. Some people think that dreams are means of solving problems while we are asleep.
Some people believe that dreams are random meaningless impulses. Those people believe that dreaming is the result of random impulses coming from the brain stem.
Some people believe that Recurring dreams are reminders of important problems that need to be solved. If the person kept ignoring a problem, the dream will be repeated again to remind him of it.
Random sounds, external noise and the change in lighting can affect dreams. Some people think that dreams are just the mind's attempt to interpret external stimuli.
Dreams can be a coping mechanism that help people deal with intense unwanted emotions. This is why a person can feel better after sleeping for sometime.