People say we have to possess a noble purpose in our life. A big dream, that is not meant merely for ourselves, but also for other people, even for our country and humanity. But when we have to deal with reality, everything can just.. Change.
Sometimes, we are just unable to create the reality as we please. Instead, it is the reality that creates us. Those who were passionate about reaching their noble cause may end up struggling in their lives, only to survive. The idealists may change to the most realistic, or even "Machiavelistic", kind of person.
For in the end, the only thing that matters to us is only our own lives. Our well-being. Our survival. Chances are we will give up pursuing our big dreams (i.e. that involves making the world a better place) when we can barely stand on our own two feet.