Hormones in boys bodies that mature as the boy does. When fully matured they cast a magical barrier over the boy. This barrier makes the boy appeasing to anything considered 'jailbait' and below. The jailbait generalizes this appeasement as attraction and then thinks that the boy is more attractive than he actually is.
Sophomore guy2: It's only because his marklons are fully developed
Sophomore guy 1: Our marklons will cause that nonsensical infatuation someday.