Why?

No matter what your status is or how well you're doing in life or anything like that - there are three things you'll always be running after: money (or wealth, power, control etc), sex (or romance, love), and happiness. 

These are things you'll never be able to fully achieve - you can achieve it for a moment, but then after some little time you'll already be looking for a better one: the next achievement. And you'll be bored with your current situation. And this will happen all the time. With anything related to those three: power, romance and happiness.*

So here comes the big boom: why do anything related to those three, if after you accomplish your goal you'll not be satisfied? 

*In the end they are all the same: the search for happiness. Which in turn is something else: the search for absence of pain. So, what you're really looking for is the absence of pain.  Why though?