@Eorl,
It is due to an absence of the pain of loss (emotional pain).
I think on the physical level, pleasure is not the absence of pain. That would be homeostasis.
One must ask, is there emotional homeostasis? I am not sure there is. Generally, despite people always answering "I'm okay" to the question, if you inquired as to the personal feelings of a group of people (and they actually gave an honest answer) you wouldn't find some without an emotional state.
I do believe that "pain" and "pleasure" are too simple of words to define the state of a person's mind at any given time. We are far too complex for this. I derive pleasure and pain from many things, but I also derive happiness and sorrow, which are rightfully different from pleasure and pain. There needs to be myriad words with myriad connotations to even begin to harness the vastness of human emotion in a communicable medium.
Sorry if I digressed, Mark. No, I don't think pleasure is merely the absence of pain, nor are they two sides of the scale. They are facets of a person's emotional personality.