system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 why do some animals die after mating?
gpt
Natural selection.

You're talking about animals where the male, most likely, isn't needed after mating.  The offspring either can take care of themselves or form some sort of colony.  Males hanging around doing nothing between mating seasons use up resources that the offspring need.  For these animals too going from newly hatched to sexual maturity also doesn't usually take that long.

Now, as to why it's not usually the adult females that drop dead after mating where the offspring can look after themselves?  They can reproduce again later.  Sure, the males could too but in the sexual evolutionary arms race a very small number of males can fertilise a large number of females.  

This is different for humans where males were important to our social groups.  They could do things like protect the group as human males are bigger and stronger than female humans, for instance.   Plus it takes years for humans to reach sexual maturity so males just dropping dead would actually waste the resources that went into getting them to sexual maturity.  

Evolution selects that which works.  It doesn't even have to work well.  It just has to work better or less shit.