Complex sentence: Clifton worked as a personnel manager during his life and interviewed close to 100,000 people, which formed much of his attitude about the delusions people place on themselves, and the greatness they are capable of.
Split into simpler sentences: Clifton worked as a personnel manager during his life and interviewed close to 100,000 people. This experience formed much of Clifton's attitude about the delusions people entertain of themselves, but also the greatness they are capable of.