Jealousy

It's just part of human nature. There's always someone better than you at something or has something you want or more than what you have. I think this is especially true when single people look at those already in relationships and why holidays like Valentine's Day are generally reviled by single people. The trick is to not let it control your life. It's natural to have some, but sometimes you need to learn to be thankful for what you do have and be content living with things you dont have. There's always usually someone far worse off than you as well.