tl;dw
Man has a natural tendency to dedicate his life to happiness, having not found a goal to his existence.
Ironically, the entire universe seems to be against this goal; it seems that it is not possible for man to be happy.
First, in the form of our own bodies; organs and neurotransmitters can be real assholes and decide that it is not time for you to enjoy yourself.
Second, in the form of the merciless outer world which has an infinite amount of force with which to crush and maul our feeble bodies.
Third, in the form of the very exchanges with our kin, which can cause us enormous damage as it is in the creation that we be social animals.
tl;dr Freud