Pleasure (n.) is derived only from contrast and only very little from the condition in question itself.
Pleasure (n.) is, for a normal functioning adult, given up as a momentary, uncertain and destructive pleasure in favor of a kind of pleasure which is delayed, restrained, but dependable.
Pleasure (n.) is sexual because it is essentially autoerotic and non-functional. Whatever is not related to self-preservation (in a darwinian sense) is for that very reason sexual.
Pleasure (n.) is an intensity which fades with domestication. The satisfying of a savage, undomesticated instinct is incomparably more intense than the satisfying of an instinct tamed by reason.
Pleasure (n.) that breeds the desire for more pleasure — how that happens is the real problem of sex.