Animals do not fight for mates because they enjoy sex, but because they want to pass on their genes. In nature, anything an animal does that benefits its survival or that of its species has evolved to be enjoyable so that the animal keeps doing it - mating, feeding and so on.
Animals themselves also enjoy sex, to the point of fighting over a mate, or of males forcing themselves upon any females they are near with.
What kinds of factors caused sex to become enjoyable to animals and humans?
