Why did Thomas say he wanted to be a Runner? What did he reveal to Chuck?

Thomas surprises himself with the declaration that he wants to be a runner. When he finds himself on a sleeping bag next to Chuck on a lawn near the garden. Quite a few of the other boys are sleeping there too. There is not enough room in the Homestead. Strangely, Thomas feels comfortable and at home in these new surroundings. This new calm startles him. Something is not right. He sits up suddenly and tells Chuck that he wants to be a Runner. Chuck dismisses his enthusiasm, telling him that he’s not even sure what he is talking about. Thomas is aware of this, but can’t shake the feeling that the Glade is familiar to him. “I think I’ve been here before”, he tells Chuck. Chuck sits up, surprised. Thomas lays down and sleeps, not sure if any more should be said about the matter.