The ID presents our instinctive urges and a lot of these urges are repressed into the conscious mind. This is to 'keep them in check' making sure that people do not say or do inappropriate or offensive things. However, the defence mechanism of repression only works to a degree and these urges eventually need to be released, potentially through dreams.

 

Dreams challenge the ego - a part of the personality that keeps us in touch with reality - and are more likely to occur at a time when the ego's defences are weakened.