Why does Gatsby throw huge parties?

Jay Gatsby is Nick Carraway's extremely wealthy neighbor. He lives in a magnificent mansion in the West Egg, directly across the sound from the Buchanan estate. When Jay Gatsby returned from the war, he became business partners with Meyer Wolfsheim and amassed a fortune in the illegal bootlegging industry. He never lost his love for Daisy, and he purchased an extravagant mansion directly across the water from her home. Gatsby knew that Daisy had married Tom Buchanan, but he still held onto his dream of one day marrying her.
In an effort to reconnect with Daisy, Gatsby hosts elaborate parties, where strangers from all over the East and West Egg are invited. Interestingly, Gatsby does not participate in the revelries taking place at his home but passively watches over his parties as a host, hoping that Daisy will attend. At the end of chapter four, Jordan Baker explains to Nick how Daisy and Jay Gatsby originally met. She then mentions the reason why Gatsby hosts elaborate parties at his mansion, telling Nick,

I think he [Gatsby] half expected her [Daisy] to wander into one of his parties, some night . . . but she never did. Then he began asking people casually if they knew her, and I was the first one he found. It was that night he sent for me at his dance, and you should have heard the elaborate way he worked up to it. (Fitzgerald, 85)

Overall, Jay Gatsby hosts elaborate parties at his mansion in the hopes that Daisy Buchanan will attend one of the parties and they will finally reconnect.

Gatsby throws huge parties because he hopes Daisy will hear of them and someday show up at one. He also hopes that someone who knows her will show up at the parties and perhaps offer him access to her. He has purchased his mansion because it is across the water from hers, in close proximity.
Although he lives close enough to Daisy to watch the green light at the end of her pier, and although he has become wealthy, Gatsby is hampered by not being in her social class (the "old money"). He is also hampered by not wanting to recognize that Tom is part of her life. When he learns that Nick is her cousin and has full access to her—and also that Nick lives next door to him—a plan to reconnect with Daisy without including Tom begins to form in Gatsby's mind.