Four sources of dreams
I. Thinking in the day carried forward even after one has cut off the awareness of one's surroundings.
II. Thoughts that arise through imbalance of elements in the body e.g. discomfort and illness. These strong sensations stir up thoughts.
III. Spirits or deceased relatives trying to make contact.
Dreams are often uncontrollable as they occur after normal awareness is cut off. How they arise is affected by our own basic nature and habits.
One who is used to having evil thoughts in the day will carry them forward into his dreams.
Similarly one with good thoughts will have good dreams.