What do East Egg and West Egg represent and why does Nick return to the west?
East and West Egg represent, respectively, the split among upper class society of old-monied, aristocratic families and the "nouveau riche" families whose fortunes were recently made. In the time setting for this novel, this division was distinct and very relevant. "Old money" was considered more respectable than "new money" and this is evident in the social depiction of life in East Egg and life in West Egg. Consider, for example, how Daisy (who lives in East Egg) considers Gatsby's (a West Egg resident) parties to be decadent and unlike the civilized gatherings she is accustomed to attending.
Nick ultimately returns west because he has become disillusioned with east coast society. He left the west, like many people of his era, in search of a richer, broader, cultural experience. After his experiences with Gatsby, Nick finds the east and its attendant lifestyle to be contemptible and lacking in authenticity.
East Egg and West Egg together represent the ongoing divisions in society. East Egg is where the "real" aristocrats live: those with older money and established credentials. West Egg is where the new money lives, and is not considered as classy.
Nick returns to West Egg because, while he has some social credentials that might allow him to live in East Egg, he is trying to make something of himself, in a way like Gatsby, his neighbor, and the western community is for those still constructing their identities.
East Egg and West Egg represent Old Money and New Money. East Egg has more social clout, respectabillity, and "taste" than those resising in West Egg.
Nick has the respectability and background to live in East Egg, yet takes a certain pride in making his own way in his life, thus associating himself with the "west eggers".
East Egg and West Egg serves to illustrate further division amongst the rich. The East Egg represents the elite of the rich. They have had money throughout generations and continue to pool in money. This is your usual grandsons of oil tycoon or 5th heir to a billion dollar company. This is old money.
The West Egg represents those who are just acquiring wealth. They are usually still in the stage of stacking up wealth. This is usually your actors and successful entrepreneurs.
The reason Nick turns to the West Egg is because of the way those who come from the East Egg love to look down on everyone that does not come from a prestigious back ground with a trust fund. Basically anyone who is not old money. Their attitude really put Nick off.