Gatsby is obsessed with Daisy. He loves her beyond any shred of rationality. When they were younger, they had a romance. Jay had not yet amassed his fortune, but he somehow hoped she would overlook the class and wealth gaps between them. She did not. She could not.
Daisy is not really the person Jay wants her to be. She is very conventional. When they parted five years earlier, she left him and married Tom, who is a crass boor despite being on her social tier.
For five years, Jay has worked tirelessly—and not too legitimately—to get rich, all in hopes of winning her back.
The reader does not know most of this when we see their meeting through Nick's eyes. The meeting is held at Nick's house, and Gatsby has also planned the next step of taking Daisy next door to his mansion to demonstrate his vast wealth. But first he has to convince her just to re-enter his life again. He is terrified of her rejection because that would mean all his efforts had been in vain.