Ok here is the complete answer in short.http://www.psywww.com/books/interp/chap01d.htm
The first factor is as Nitewalk has stated, the normal reasons for forgetfulness in waking moments, happen to the dreamworld to. The second factor is the human mind tend to forget the one time events that occur and focus more on the repetitive (which is why people's long term memory over a single event can not be trusted as well as their long term memory about a routine event) Of much greater significance is a third cause of forgetting. In order that feelings, representations, ideas and the like should attain a certain degree of memorability, it is important that they should not remain isolated, but that they should enter into connections and associations of an appropriate nature. If the words of a verse of poetry are taken and mixed together, it will be very difficult to remember them. "Properly placed, in a significant sequence, one word helps another, and the whole, making sense, remains and is easily and lastingly fixed in the memory.
Is someone elses turn to pose a question