The ability to understand people's motivations.

If you don't understand people, they become arbitrary. You'll live in a world where people screw you over for no reason, and you'll waste a lot of energy being angry and depressed. You might also think people's actions have something to do with you when they don't. This will mess with your self-image and hinder your interactions with other people.

You'll have a lot of trouble resolving conflicts because good conflict resolution requires both parties to see the other person's side. This will cause you to lose friends and put you at a disadvantage in your career. It also makes people hate you.