The most difficult people to deal with are those who never truly grew up—who carried all their childhood bad habits into adulthood. Although they are grown, they haven't learned to control their temper, still pouting over everything, and refuse to restrain their desires, unleashing tantrums whenever they please.
These are the people who can't connect emotionally, who remain trapped in their egos, constantly seeking care without ever offering it in return. They cling to their rights yet pretend not to understand when you bring up responsibilities, the "shoulds" and "musts."
In the end, the hardest people to face are those who act like spoiled children, still trapped in an adult body.