To understand Gatsby's proverbial roller coaster of emotions during his reunion with Daisy, it's important to know the events in the years leading to the reunion. Gatsby and Daisy haven't seen each other for years. In the time that they have been apart, Gatsby has put Daisy on a pedestal and has created an image of her in his mind that is probably far more ideal than the real Daisy. He has set his expectations incredibly high. In meeting her, he feels a complete mixture of emotions including anxiety and discomfort, elation, and finally slight disappointment.
The awkwardness at the beginning of the reunion is palpable. Gatsby has been waiting for this moment and does not know how to handle himself. He tries to look cool and calm but fails, managing to knock over a clock. "He turned and caught it with trembling fingers, and set it back in place." He can't seem to get himself comfortable in Daisy's presence. This is a Gatsby readers have not seen until this moment. Soon after, he says to Nick "'This is a terrible mistake.'" The reunion isn't going the way he has pictured- likely for five years- and his nerves are getting the best of him.
Nick leaves this awkward scene and later returns to find a completely different atmosphere. Gatsby's attitude has shifted now to one of complete elation. "He literally glowed; without a word or gesture of exultation a new well-being radiated from him and filled the little room." Now, by no coincidence, the weather has cleared up and and sun is shining, reflecting Gatby's happiness at being reunited with his lover after so many years.
Finally, just as anyone experiences when he finally realizes his dream, Gatsby feels, from Nick's observation, a sense of disappointment at the reality of it all. He has been building this moment up in his mind for five years, and it unsurprisingly does not meet his expectations. When he observes the green light that has for so long represented his dream of Daisy, he discovers that it is now only a light. Nick observes that Gatsby has a look on his face that looks as if "a faint doubt had occurred to him as to the quality of his present happiness." It seems, from Nick's perspective, that upon being reunited with Daisy, Gatsby isn't sure if the happiness he is currently experiencing is the result of being with Daisy or the result of realizing a five year-old dream. Though he still seems joyful in her presence, there is a shadow of disappointment on his face.
Gatsby's change in emotions upon seeing Daisy is understandable and many readers can probably relate. He has had a dream for years and has put a lot on the line to achieve that dream. In the beginning of chapter five, he is incredibly nervous. When the nerves subside and he becomes comfortable he is elated, and finally the elation wears off, giving way to a slight disappointment.