A friend has written a really good essay:
I carry with me a great sense of sorrow. There are moments when it consumes me, shaping all my thoughts and perceptions, and seeking more instances of sorrow to add to its weight. At other times, this sorrow rests just beneath my consciousness, when I am fully occupied in an activity or experience, or have reached a rare state of stillness, or when I am feeling joy (though I believe my sorrow is fully present in my joy.) Even when it rests beneath my consciousness, though, I can call it up at most any moment, and indeed I sometimes do, for I often know a greater sense of identity or truth when I call upon sorrow, as long as the sorrow is pure. There is such a thing as impure sorrow, I think. It is the kind which leads to, or takes the form of, depression. It becomes impure because something within is fighting against it, unwilling to let it fully enter and reveal a new layer of self or reality above which I am currently residing.