1) birth and 2) death.
In the first case it's generally because a male and a female animal mate that conceive new lifes. If this is done by human beings, it's not exactly like the rest of the wildlife world. We do not mate just for reproduction but also to glue the union with our opposite sex partner to ensure the surviving of the offspring. Because of the pleasure involved in human mating we got a much more interesting sexual life than the rest of the animal realm, and, mostly the males, tend to forget that somehow, sometime, somewhere he is going to conceive a new life sooner or later with his female partner. Science tends to establish the "conceiving event" at the moment that the female egg starts his first division. But this is a female point of view. The male point of view... Jeez! I got lost! :D Please help me!