@ImMachiavellian,
There is no purpose.
The reason we THINK there is a purpose in life is because we CAN think. It is nothing more than a sentient question that lays dormant in our minds.
BUT, if you are implying "what is the reason of natural life" as in "everything that is biotic and exist" has a simple explanation.
Why are birds alive?- to reproduce, evolve, stay alive.
Why are worms alive?- to reproduce, evolve, stay alive.
Why are monkeys alive?- to reproduce, evolve, stay alive.
and even to the extent- Why are parasites alive?- to reproduce, evolve, stay alive.
The answer is simple, our purpose in life is to reproduce, evolve, stay alive as it is with every other organic in our world, probably even the universe.