Jealousy
Jealousy is a green monster that sneaks up on you when you least suspect it.
It breaks up friendships and relationships. Even when you don't express it, it still affects you. A twinge of jealousy towards your friend can lead to hours of contemplation and guilt later on. Now where does this jealousy come from? Is it built in to humans the way breathing is? Or is it a learned trait from the environment? Natural selection and survival could be the root of the problem, but then it should be able to be turned off when it's not necessary.
I believe that jealousy is fostered by the environment. In this day and age, everything is competitive from college to careers. Everyone is fighting for survival no different than how is was in the prehistoric days. It's just all hidden now. Jealousy has adapted, it now wears a mask. A friendly face that behaves itself. When triggered by a threat, its ugly green face peeks out and bites the threat in the nose. Or, it just remains inside, building and building, until one day, it releases.
Not everyone is jealous, and some more than others. Some people are just easygoing or have really big hearts. They aren't all about competition, and haven't been sucked in by society's pressure and expectations. I wish I could say I am one of those people. But, I'm not. My green monster lies somewhere deep inside. I may feel the jealousy, but I know it's wrong, so I shove it deeper until I no longer feel it.