dreams are supposed to be the random firing of synapses in the brain...theres is only the brining up of past thoughts feelings experiences etc. from this, it takes a chemical movement to create a new connect/synapse, which is something we do not do when we dream...dreaming is purely electrical in nature (electrical = immediate and short term, chemical = long term), thus unless you start thinking with your concious mind and make an effort to make a chemical reaction, the electrical differences in your synapses dissapates as new electrical signals quickly come in...thus we forget them as we wake up, aka the more we intake (sight sound etc)...hope this helps