You dream so much due to an accumulation of thoughts in your subconscious when you are awake. These thoughts are suppressed by your conscious mind and when you sleep (temporary loss of consciousness), the thoughts will surface.
1 Additional Answer
Most experts say that the stuff of our dreams is just a replay of the things that we witnessed (consciously or subconsciously) during the few days preceding the dream. Others say that our dreams are a fortune teller of some sort.