が → を with ~がる verbs

When an emotion adjective like 怖い, 欲しい, or 寂しい takes the suffix ~がる to become a verb describing someone else's feelings, the particle marking the trigger of the feeling shifts from が to を: 犬が怖い → 犬を怖がる, りんごが欲しい → りんごを欲しがる. The verb gains a transitive frame.