Teenagers rebel against their parents under normal circumstances. Sandy has traditionalist parents in a time before Dr. Spock or psychology-based notions of parenting. He is also being actively recruited by a political organization designed, as Herman pointed out earlier, to separate children from their parents. From these other adults who aren't his boring, restrictive parents, he gets praise, freedom, encouragement of his talent, exciting opportunities, the promise of a new life. Taking advantage of the biology of adolescence is what makes these programs so insidious and why it's possible to, for example, convince comfortable middle-class teenagers to abandon their families and join ISIS.