Why is Daisy full of unexpected joy?

Daisy was full of unexpected joy because she was surprised at Gatsby's presence, but she was also as happy to see him, as he was to see her.

They were sitting at either end of the couch, looking at each other as if some question had been asked, or was in the air, and every vestige of embarrassment was gone. Daisy’s face was smeared with tears, and when I came in she jumped up and began wiping at it with her handkerchief before a mirror. But there was a change in Gatsby that was simply confounding. He literally glowed; without a word or a gesture of exultation a new well-being radiated from him and filled the little room.