[Quiet Man Cometh]

I think a good explanation of how stories resemble each other is to take a lot at Joseph Campbell's monomyth idea. He made a study of world relgions and took the similarities between the to create what he figured were the essential building blocks of all myth, sort of. Don't take me word for word on that. He has a piece called "The Heroes Journey" which details the stages of an adventure. For example: the call to action which sends the hero out on his quest; Mentor, or the figure who guides the hero's efforts; the quest for a needed object to solve the current problem.

The call to action might be in the form of a wizard with a pointy had that arrives on your doorstep with a bunch of dwarves in tow, or the head of a spy network that sends out his best agent on a suicide mission, or something happens to the protagonist that makes him want to leave and do something (ie, revenge for death, need to help sick family member, etc). Obi-Wan Kenobi makes a dandy Mentor figure to guide Luke on his quest. In this sense, all stories are made up of common elements which are coloured over to flesh out the story into something entertaining rather than a bland narrative.