The nucleus accumbens is a rather small structure in the mammalian, and therefore human, forebrain, located in the ventral striatum. It has various functions, but one of the most prominent of these is motivation.
Yes, humans seek pleasure, but without the drive to seek it, even pleasure is void of meaning.
Motivation is central to life. In the very beginning was, and is, motivation. Motivation does not have to be conscious, as even worms exhibit it.
And you may just want to imagine the nucleus accumbens as a worm. Because it still has a degree of neurological autonomy that makes sense even if it weren't expanded by all those higher brain functions.
The nucleus accumbens, along with nearby structures like the thalamus, is at the basis of reward-driven motor processing and memory. Even though a great deal of data is stored in the cortex, its emotional encoding happens in the subcortical brain, particularly the nucleus accumbens and the thalamus. And the emotional encoding of data is what makes it relevant for motivation.
Basic memory functions in the subcortical brain include spatial and positive-negative memory encoding. These two are needed to seek nutrition (eat) and to avoid becoming nutrition (being eaten), as well as to evade other dangers to an organism's existence.
One level up, but still very much tied to the subcortical brain, is the motivation to procreate, which is sexual in all higher forms of life.
Inasmuch as seeking nutrition is a banality in modern life, and inasmuch as a human with a functional brain will succeed in avoiding most dangers to life and even a great deal of discomfort, sexual motivation rules ever more supreme.
In a developed human society, individuals pursue all kinds of goals. But I credit Sigmund Freud with advancing the awareness that many human endeavors are merely substitutes for sexual motivation, or alternative routes to achieving sexual success.
