Dreams are mostly forgotten because you have a dream in a certain state and then you pass on to another state. For example, when you sleep, the body is asleep, the vital is asleep, but the mind could be still active. So your mind begins to have dreams, its activity is more or less coordinated, the imagination is very active and you see all kinds of things, even take part in extraordinary happenings... After some time, all that calms down and the mind also begins to doze. The vital that was resting wakes up, it comes out of the body, walks about, goes here and there, does all kinds of things, reacts, sometimes fights, eats, etc. The vital is very adventurous. When it is heroic it rushes to save people or destroy enemies or it makes wonderful discoveries. But this pushes back the whole mental dream far behind, you cannot remember it because the vital dream takes it place, it gets forgotten, rubbed off. Then something else wakes up. Let us suppose that it is the subtle physical that goes for a walk. It starts moving and begins wandering, if you wake up without stirring you remember that.