Selfish: Holding one's self-interest as the standard for decision making; having regard for oneself above others' well-being. Selfishness, we all take part of it at one point or another. It's something some try to stay away from, while others simply don't even think about it. "Selfishness is mankind's fundamental defect" (http://www.believers.org/believe/bel221.htm) Selfishness is the root of many problems. It can cause broken hearts, relationship departure, chaos within the hearts and minds of others around you, and so much more. Selfishness is putting yourself first, thinking about what you desire or need rather than anyone else.
Two characters in The Great Gatsby that posses selfish attitudes are Tom and Myrtle. Even though they are both married, they choose to follow their selfish desires and give in to the temptation of cheating. Even though they are together, Tom realizes what a fool he was for even getting into a relationship with Myrtle in the first place. He sees that she was just as satisfying as Daisy was and is upset with the fact that he has to be a partner in two different relationships now.
Myrtle, on the other hand, is jealous of Daisy because she has to share Tom. She's being selfish because she's upset with the fact that Daisy has Tom too. She's also selfish because she takes his money without a care. Myrtle saw a man selling puppies on the street and told Tom to buy her one. Instead of asking him if it was okay, she assumed he would buy her one. "It has been said that selfishness is the great curse of the human race. Certainly to live selfishly is to be shut off from life; to be blind to the perspectives, needs and concerns of others."(http://www.hypnosisdownloads.com/personal-skills/selfishness)