At the open of The Great Gatsby, we learn that Nick Carraway has recently moved to West Egg to seek work in bonds. His next door neighbor, Jay Gatsby, lives in a huge mansion and throws wild parties every weekend. Across the Bay, Nick goes to visit the home of his cousin Daisy Buchanan. Which Nick attends one of Gatsby's parties, he takes a special interest in Nick and begins using him to find a way to connect with Daisy.
As it turns out, Gatsby and Daisy were lovers many years before. Even though Daisy has married (to ensure her financial security,) Gatsby never moved on from their romance. It is so significant that Gatsby's house is located across the Bay from West Egg, and specifically across from Daisy's house, because he wants to be close to her after all this time. Though Gatsby had not made attempts to see her prior to Nick's arrival, he would often look at a light at the end of Daisy's dock and admire it as a symbol of her. He could not see her physically, but he could think of her fondly and know she was on the other side of the Bay- sometimes thinking of him, too.