Evolutionary motivation for the existence of pleasure? It seems like pleasure goes beyond needs. What is its purpose? Is it supposed to make you not do anything, because of how good you feel?
Are there animals that don't feel pleasure, but only have needs to satisfy, and how would their CNS differ from human/primate CNS?

My main question: What is the motivation for there being some sort of pleasure, as opposed to simply the absence of a certain need? In the case of an orgasm, why exactly does a person feel very good for a while - why doesn't he just go directly into a state of neutrality, with there no longer being a need for sex? If a person defecates, why is there a momentary pleasant feeling - why doesn't the person simply go into a 'no longer have the need to defecate' state?

It really seems that there is an unnecessarily large amount of pleasure, as opposed to a mere lack of need.

It seems like there are 2 mechanisms that motivate action - the need to satisfy an unfulfilled need, and the need to achieve a reward. Is this the case? If yes, what is useful about this? It seems like a the 'need to satisfy an unfulfilled need' by itself should be good enough.

EDIT:

For example, we have an instinctive need to not feel like starving, or to not feel extremely tired - we simply don't want to feel that way. However with some things, it seems like there's more than just a need - with sex, there is a 'need' to have sex - a male who hasn't had sex for long enough, will simply want to have sex, so that he gets rid of the feeling of wanting sex - but there is also the motivation of expected pleasure from the orgasm. Similarly, I'd say that there is some sort of pleasure from food.

I think this is true (that there are 2 motivating factors, not just one), and I think that it's quite weird in some sense. Why isn't there just a need - why doesn't a male after sex simply stop having the need to have sex? I think I've read something about the pleasure having something to do with encouraging certain types of behavior, but I really don't know.
First a couple of definitions:


Positive reinforcement occurs when an event or stimulus is presented as a consequence of a behavior and the behavior increases.
Negative reinforcement occurs when the rate of a behavior increases because an aversive event or stimulus is removed or prevented from happening.


Learning may occur as a result of negative or positive reinforcement; ie, through the elimination of a need, or through the pleasure of reward.  Traditionally, these have indeed been thought of as two complementary mechanisms as you suggest, but the modern view is that they are actually synonyms.  They are different ways of looking at the same thing, like describing a glass of water based on how full or how empty it is.

For example, eating can be described as the elimination of hunger (negative reinforcement) or as the pleasure of taste (positive reinforcement).  The choice depends on the context of the situation, but it is always possible to describe any reinforcement as both negative and positive.

As for why the feeling of pleasure exists at all, you can think of it this way: If an action only results in the elimination of a need, then there is no motivation for the subject to do any better than that.  We can fulfil our hunger with rice-cakes, but we are motivated to seek high-calorie foods because of the pleasure they give.  This was an essential survival mechanism for our ancestors.