"Tuck almost never smiled except in sleep."
When Mae and Tuck are introduced, they are waking up. Mae is instantly happy and super excited to see her sons. Tuck wants to go back to sleep. He does not want to go back to sleep because he is tired, though. He wants to go back to sleep in order to re-enter the good dream he was having. The dream is the reason that he was smiling in his sleep.
"I was having that dream again, the good one where we're all in heaven and never heard of Treegap."
It's too early in the book at this point for the reader to know what Tuck is talking about. Why would he want to have never heard of Treegap? It sounded quaint enough in chapter 1. Is his life really so bad that he strongly wishes to be in heaven immediately? At the end of the chapter though, the narrator drops the bomb. Mae, Tuck, and their sons have all looked the same for 87 years. Tuck is an immortal. Sounds cool, but Tuck has had enough of it. He wants to age, grow old, and be "normal." That's what happens in the dream. That's why it makes him happy and smile. He gets to experience his deepest desire.