Jealousy

Jealousy is an emotion and typically refers to the negative thoughts and feelings of insecurity, fear, and anxiety over an anticipated loss of something that the person values, such as a relationship, friendship, or love. Jealousy often consists of a combination of emotions such as anger, sadness, and disgust. It is not to be confused with envy.

Jealousy makes Daisy keep bringing Tom along to Gatsby’s parties even if she is truly devoted to Gatsby. She flaunts her love affair in front of Tom. Tom is having an affair. Everyone knows about it. She wants her revenge. However, she never plans on leaving him. Daisy after all is human. This explains why when Gatsby forces the confrontation, she backs down.

At the hotel room in New York, Tom strikes Myrtle across the mouth, and he is demeaning toward George Wilson at his garage. Later, he manipulates George by playing upon his hatred and jealousy of Gatsby by leading Wilson to believe that Gatsby, as the lover of Myrtle, is responsible for her death -- a strategic play in order for him to "win" and save Daisy's reputation.

Source: http://www.echeat.com/essay.php?t=27130