Could it be because it reminds us that we are alive, of our mortality, of our individual souls- which, after all, we are too afraid to surrender but yet make us feel more miserable than any other thing? But isn't it also pain that often makes us most aware of self? It is a terrible thing to learn as a child that one is a being separate from the world, that no one and no thing hurts along with one's burned tongues and skinned knees, that one's aches and pains are all one's own. Even more terrible, as we grow old, to learn that no person, no matter how beloved, can ever truly understand us. Our own selves make us most unhappy, and that's why we're so anxious to lose them, don't you think?