Marjorie

Marjorie is a simple being. In all of her life, all she does is move back and forth in a tube filled with goo. Marjorie is a humble collection of cells. That happy little village of cells lives off the goo in the tube. Whenever the goo is hot, the little cells work and work, and the product of their work, expelled in puffs, propels the whole in some direction. The hotter it gets, the more frequent these puffs are. Life is pleasant for Marjorie.

Now, being temperature the only thing we can tell Marjorie craves for, we decided to stick a candle under the pipe to make her happy. Far away from the candle, Marjorie swims back and forth at random, with wide gaps between the puffs that send her in either direction. In this dark extremity, she is starving. Her cells are slowly dying away. Fortunately, thanks to aeons of natural selection, the cells of her kin have developed this ability to emit a beneficial substance that stimulates cellular growth whenever there is a sudden increase in their activity. Because the walls of the cells only absorb substances when they're puffing, only the active cells at the moment of the increase in activity will absorb it. So if a cell is puffing at the time when an increase of temperature happens (which causes an increase in cell activity), the activity of that cell is reinforced. In this way, the cells whose activity brings the whole closer to the candle will work more and more, whereas the other ones will work just as much. They will swim towards the candle, away from the darkness where they wanted as the collective of cells, otherwise known to us as Marjorie, experiences pleasure. That is what pleasure is: the activation of a mechanism by which a certain behavior is reinforced. The more general process is called learning. Marjorie has learned that swimming towards the heat is good.
It is important to notice that this type of learning, pleasure, is pure positive reinforcement. If the candle were placed at the middle of the pipe, Marjorie would happily swim past it until the cells swimming in the other direction get equally reinforced.

Her cells, unfortunately, do not deal so well with excessive heat. If it's too hot, her cells start to die. Again, thanks to generations of trial and error, whenever a cell dies, it emits a substance which is absorbed by the puffing cells. However, this time, the substance actually kills the cell, or at least reduces its activity. It's as if when a cell died, it inadvertently exerted a post-mortem revenge on those who caused its death. In practice, this results in a decrease in the activity of the cells that swim towards the heat whenever the heat becomes too excessive and starts to damage the cells. The whole, Marjorie, experiences suffering. It is also a learning experience, but a negative one. That's what suffering is: the activation of a mechanism by which a specific behavior is inhibited.
But this reaction is not enough. Suffering only teaches not to do, it doesn't force Marjorie out of her present condition where if she stays, she will slowly die, cell by cell. Though let's not forget that as she shakes around that spot, any increase in activity will release beneficial substances which will reinforce the motion away from the excessive heat. Marjorie will eventually find a stable position at the sweetest spot between fire and ice where she will live on to grow and prosper, reproducing all the way to a beautiful dinosaur.

In our own brains, this basic behavior of elements reinforcing what helps them and punishing what harms them is enough to explain intelligent behavior. Marjorie was meant to point out how suffering and pleasure come about naturally. Therefore any adaptive system experiences them.
In our own brains, more and more systems have developed on top of each other. We not only have systems which reinforce the increase in the cellular activity but we also have systems which directly react to the variations of pain conveyors, to reinforce behavior which minimizes the pain. That is why you feel pleasure when pain ceases. That is also why you seek more and more that behavior. It is the underlying dynamical situation which translates into pleasure in your own subjective experience.

On the other hand, if the system reacts positively to the substance emitted by the cells when they die, the behavior which led to that will be reinforced and a self-destructive trajectory is undertaken. In your own life, you would perceive that as cutting yourself with a knife and doing it more and more often, instead of less and less as it usually happens. You would feel pleasure instead of suffering. Masochism is just that. Pain does not have to mean suffering. It's how the system reacts to it which dictates the way it's experienced.