Passing on your genes.
It's built in.
Males display, females choose.
Finch: song and trace of red
Peacock: tail feathers
Giraffe: long neck
Irish Elk: Big antlers. Females selected large antlered males - resulted in elk with bigger and bigger antlers
Rams: Horns, head butting
Gabralter McCacks: Sex once an hour, female screams during sex - arouses male, helps him ejaculate.
Humans have longest penis in mammal world. Necessary because of upright posture of the female.
To be as physically mature as a chimpanzee at birth, human baby would have to be in womb for 21 months.
Evolution ejects baby at 9 months because of size of brain and mother's birth canal. Thus mother and father are required to stay together longer.
Evolves "Love" and the need to stay together to nuture the baby.