When a man life becomes complete
The question "why?" with regards to existence comes from theistic thinking. Even some scientists cannot free themselves from this kind of thinking. It is a result of believing that there's some ultimate entity/force in the universe that decides a purpose for us.   

Buddhism doesn't subscribe to ideas like that. Buddhism teaches about causes and effect. It never talks about an inherent purpose or something written in the stars with regards to existence. Because there is none. But that doesn't mean that there are no goals worth aiming for. In an ultimate sense, end of suffering is the goal that is worth striving for. 

If your head is on fire, trying to put off the fire is the smarter thing to do. Not to ask "why me?".