Why all people in the world can dreaming?

Answer 1

One of the most important issues to our survival is sleeping. Then, our body is cleaning up; like: detoxifies itself. Some of the results include sweating, going to the bathroom, etc., but also, our memory is classifying the inputs of the day, etc. When the restoration finishes, the brain works out the past and organizes the future. Among those "calculations" some brain part is preparing the dreams!
And the above is true for all living organisms (people and animals)!