Because every single one of us is a totally unique entity, that which gives us meaning in life will differ from person to person. Sure, it seems totally obvious, but try and think about what this really means...
We ask, "why are we here?" For each of us, there is a different answer for the stuff beyond simple survival and well-being. Some, especially those who expound upon the virtues of organized religion, would have you believe that there is one true answer to this question for all peoples. I believe there is not. Why should we all relate to life, to the creator if there is one, to our hopes and dreams and ideas, to what drives us and moves us and makes us feel truly alive in those rare perfect moments, in exactly the same way as everyone else?
We are a multitude of shifting points on shifting spectrums, not static things in categories.
There are more kinds of sheer aesthetic pleasure than I can possibly imagine. The religions change and shift and merge and fragment, the ones with one person and the ones with millions. We pour out our souls in art and music and any form of expression imaginable, we explore and seek (and sometimes find!) and wonder what's over that unexplored horizon, we pursue the feeling that everything is right with the world, we share it all with friends - and each of us does it differently! Each of us sees everyone else doing everything in a unique light! There are no answers but your own! There is no meaning to life but what you give it!