Nihilism
It seems obvious to me that life is without a "purpose". All proposed such purposes I have heard of simply offload the responsibility to another source that humans contribute to. If you put your faith into one of these higher causes without further consideration, you now have a purpose in life. Looking any deeper, however, may cause you to wonder what purpose this higher source holds. Maybe it offloads the responsibility even further, but in the end the search for a "purpose" is unfulfillable. Does a gear that drives a useless machine really serve a purpose?
I do not see this as a reason for demotivation. If anything, it's quite amazing the amount of complexity that has been created as a result of life sustaining itself. Is there any one purpose that could be worthy of such a magnificant cycle?