The reunion between Gatsby and Daisy is pivotal to the story because up until this point we have only heard Gatsby talk about it. We are not sure if he really knew Daisy and had a relationship with her. We just have his word to go on, but when they have their meeting, we can see that there are feelings there, the kind of feelings that have been there for a long time.
Gatsby is trying to get Nick to arrange a meeting between himself and Daisy. When Nick finally agrees, Gatsby is beyond thrilled. When it is time for the meeting, Gatsby isn't there at first. When he does arrive, he is soaking wet from taking a walk in the rain. When Daisy and Gatsby set eyes on each other, Nick had enough sense to leave. At first the reunion is very awkward. Gatsby is extremely nervous. In all reality Gatsby is finally being his real self. He is not putting on a front for other people. His real person is exposed. The awkwardness finally goes away, and the two of them have a great reunion. They spend a great deal of time together. Gatsby admits to watching the green light from Daisy's dock, knowing that it is her house.
"If it wasn't for the mist we could see your home across the bay...You always have a green light that all night that burns all night at the end of your dock."
After this meeting, the plot focuses on the romance between Gatsby and Daisy, and the tragic consequences that follows. This is why chapter 5 is so pivotal to the story. This chapter takes things in a whole new direction and sets up the reader for the tragedy that is to follow.