Why we, quite erroneously, think we are a "self": memories -- including those pertaining to what has most recently occurred in the immediate environment, or within our own conceptual thought spaces; both are a kind of RAM. Without being able to remember that I just typed the last portion of this clause, I would not have the illusory perception of being a discrete self who never changes.
On the contrary, every time I breathe, the molecular composition of my finite body is altered. Meanwhile, qualitatively identical copies of the collection of drives which aid in the continuation of the process of self-perception are found in millions of other living organisms, many of which suffer horribly for no reason whatsoever throughout their lives. If we as "selves" do not exist and are merely a collection of copies of feeling reflexes, shouldn't we concede that our lack of memory for the horror that takes place on a daily basis on this planet is irrelevant to the fact that copies of "us" continually experience it?
Also remember that feeling reflexes receive data along a continuous gradient, but that there are two separate gradients, each contained within its respective, discrete state. In essence, then, sentience is a binary system in spite of the nuances that can be found on both sides.
In spite of this, there are no hard, physical walls separating the molecules that make me who I "am" from those that make you who you "are." It's likely that some of the molecules currently representing my circulatory system used to be, prior to my digesting them, minerals found in various foods. Before this, perhaps millions of years ago, those same molecules helped to represent the wing of a pterodactyl. I may not have the capacity to remember the harm that has been ongoing for hundreds of millions of years on Earth, but each organism is not a universe unto itself; it is an aggregate of drives, propensities, senses, and mental capacities -- all copies, and all of the same general biochemical composition. An individual organism is merely one location in which the unified phenomena that we can collectively refer to as "suffering" take place.
To put it more directly, the universe itself is what suffers; only memory makes us think otherwise. There aren't any physical organisms so much as there are organisms as genetic processes that use physical material in order to synthesize, metabolize, replicate, etc. Take away our memories, and it will become quite obvious that all of the feeling reflexes on Earth are equally important, not at all different or separate. Connect all the memories to create a super organism, and it will become even more obvious.