Evolutionary motivation for the existence of pleasure?
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.