In The Great Gatsby, Jay Gatsby throws parties to fill a void in his past; to exercise his present power; and to impress the one woman he had ever loved, for whom he wasn't good enough. Jay Gatsby began life in humble circumstances, tried to improve his situation, met wealthy, pampered Daisy and fell in love. She didn't marry him because he wasn't good enough and could neither appreciate nor provide all the comforts she was accustomed to.
When Gatsby made his fortune through dishonest means, he began surrounding himself with the pomp, luxury and social acceptance he never had and which Daisy had demanded of him. He entertained a secret hope that Daisy would find her way into his world again. He threw lavish parties to create what Daisy demanded and to exercise the power of his vast wealth, knowing all the while that people had heard rumors about his business dealings and the source of his wealth and didn't really think of him as a friend: To the social circle, Gatsby was only a good time, but the people partying at his home gave him what he was looking for anyway.
Gatsby was trying to get Daisy over to his house in someway. He used the parties to do this. Also you know that weird part where Gatsby is trying facing the light with his arms up. He is really looking for Daisy. Thats what my english teacher told me.