Animals make choices based upon their knowledge of the environment and their own phenotype to maximize their ability to reproduce and survive. ~ English zoologist George Lovell
Too much has been made by evolutionary biologists that life’s purpose is propagation. The repertoire of animal behaviors and their motives are too expansive to characterize life so singularly.
That said, breeding plays an outsized role in every individual inclined toward sex, precisely because the desire is so keenly felt. Biology impels the mind. Many animals sacrifice their lives to bring forth the next generation.
The desire for a mate is especially compelling for animals where the partnership extends beyond mating. Pet fish left alone in a tank after a partner dies have been known to commit suicide by leaping from the water and dying on the floor.
Monogamy-minded zebra finches are choosy about their mates. Like humans, finch mating preferences are individual, based upon life experiences and personality makeup.
Zebra finch fathers participate in childcare. Successful upbringing of offspring depends upon a couple being happy together. This too resembles the human experience.
Then there is the joy of sex, which varies widely. For many, however driven to enact it, sex is a mirthless moment. At the other end of the spectrum, the compelling stimulus to sex can be a biochemical reward: a pleasure that often accompanies the act, or at least its prelude. Sex is fun. This especially tends to be the case in gregarious animals.
Biochemical structures evolved many hundreds of millions of years ago that provide positive reinforcement for life-engendering behaviors. Emotional payoff for sex is adaptive to species lifestyle; just another life-history variable. (A life-history variable is a trait which may be considered a trade-off from an evolutionary perspective.)