Why was Thomas so disoriented and confused in Chapter 1? Who was he finally greeted by? Why?

Thomas awakens in a dark metal room which he quickly realizes is an ascending elevator. He can only remember his name but nothing else about his life. He has no idea where he is or where he is going. Most importantly, he has no idea why any of this is happening. Thomas is perplexed by his memory loss. He can recall images of the world but cannot remember anyone in it. He cannot remember his parents’ faces or a single conversation he has ever had with anyone. One boy greets him with words Thomas knows he will never forget: “Welcome to the Glade.” The boy's name is Alby.