Why Do Two Mammals Have Sexual Intercourse?

Nature is a constantly fluctuating pattern. You can only designate particular wiggles in a pattern arbitrarily. So just as there are no point instance in the curve, so there are no events in nature. It becomes nonsense to speak of one event causing another. What you really mean is that the two events which you speak of as being causally related are simply two parts of the same event. They go with each other in the same way as this with that. The relationship is not causal. It is mutual.