Why do we have them? Why do some people react with anger and when some react to the same event with happiness? I understand that "without bad you wouldn't appreciate good" but if we didn't have any emotion to begin with, would we even be able to tell the difference between "good" and "bad"? Would there even BE a good and bad?
I really think emotions, just like all of our other little quirks, are another by-product of our highly evolved ability to discern patterns. When we first started developing this skill, it was just bad and good. Bad things hurt, good things felt good. We knew that predators, pain and death were bad, and food and sex were good. That's basic survival, and as we developed the ability to make ourselves more and more comfortable, and thus thrive, that ability diversified and we developed more and more complex emotions. That's how I think of them, anyway.