Dreams

What are dreams and where do they come from? How come some dreams come to pass and some do not? Can we chose our dreams?

I have had dreams in the past which came to pass, or through which I got to know about some happening in someones life which I had no other human way of knowing. These have typically been very far in between, and quite scary for me. Now, in the space of one week, I have had two such occurrences. I did not ask for this "gift", and I do not want it. But I would like to understand it.