I think this song is just about a great loss in general, perhaps someone you love has gone away forever, and you will never see them again, or an oppertunity has passed you by, and you feel that empty space inside you.
Or maybe it has a more literal meaning, that perhaps you dream and it's such a great dream that when you wake, you feel upset because you miss whoever it was in your dream that made you feel that way.
I think this song is just about a great loss in general, perhaps someone you love has gone away forever, and you will never see them again, or an oppertunity has passed you by, and you feel that empty space inside you.
Or maybe it has a more literal meaning, that perhaps you dream and it's such a great dream that when you wake, you feel upset because you miss whoever it was in your dream that made you feel that way.