What motivates Holden in chapter 20?

Holden's primary interest shifts from Jane Gallagher to his sister, Phoebe. He even seems more preoccupied in seeing Phoebe than in his imminent date with Sally Hayes, for whom he has little more than contempt. The fascination that Holden has for Phoebe seems part of a longing for childhood. Holden resists change; he dislikes trips to the museum precisely because their static nature reminds him how much he changes at every visit. Holden seems to fear change and maturity, giving great sentimental weight to childish pleasures while fearing the qualities that mark adult life.