Why do we feel pain?

This past week had an amazing episode of Grey's Anatomy, AS ALWAYS!!! The topic at hand tonight was pain. We experience pain in many different ways, whether it be falling and scraping our knee or losing a loved one. We have sensors in our bodies which allow us to feel this pain. But what would happen if we didn't feel this pain? Would our lives be better without the hurt we feel in life? In tonight's episode, a young girl named Megan came to the emergency room with a large laceration on her leg. Upon examination of Megan, another large laceration was found on her arm, but this time it had been closed up with staples one would find in a staple gun. Turns out she stapled her own arm after cutting it open. What we find out is Megan has a genetic condition in which she feels no pain. Because of this, she thinks she has super powers. She often tells kids to hit her as hard as they can in the stomach to prove she feels no pain. The numerous blows to her abdomen, however, have caused her to bleed internally. So it begs to question, why do we feel pain? Do we feel pain so we know our boundaries? Is it because it is the opposite of pleasure. There comes a time when pleasure can be too much, and when this happens, we need to feel pain. On the flip side, do we have to feel pain to know what pleasure is? Without pain we would never know when we experience the good life has to offer. So why is it that something that seems so necessary in our lives to avoid near deadly situations like Megan's, hurts so much? And why is it that the things which are not good for us feel so good??