What does Miss Thomas tell Bud to keep in mind? Why does Bud feel strange when she tells him this?

In chapter 16, after Miss Thomas has told Bud that he can stay with her and the band, she also tells him that she considers him "a godsend," and she tells Bud that he must "keep that in mind all of the time." This is obviously really important to Miss Thomas, and so she takes Bud by the arms, looks him in the eyes and says, "Really, Bud, I want you to always keep that in mind . . . I don't want you to forget what I'm telling you."
Bud says that this makes him feel "strange." This is most likely because he hasn't been told very often before that he is "a godsend." Miss Thomas tells him this, and insists that he remember it, because she doesn't want him ever to feel like he is a burden on her or his new family. Bud is simply not used to people being so kind to him. He is used to being made to feel like a burden. He has, after all—before he met Miss Thomas and the band—been shifted from one foster family to another. It is, therefore, an unusual, or "strange" experience for him to be told, and so earnestly, that he is not a burden but "a godsend."