Nearly ever person alive today has thought this question at least once in their lives. The direct answer is not that important, however the indirect ways which come along, are important, at least for me.
Since July 20, 2003 I exist because my child does. She gives my life purpose and meaning. Something to look forward to, something to hope for, and something to treasure for the rest of my living days.
There are people who exist today, that by choice or circumstance don't have children no matter their age or relationship with others. Their purpose of life may be the one they love the most, or themselves, or perhaps even an inanimate object. This is their lives and I'm no god; I haven't the power to judge or rationalize their lives.
Not to say these lives are worthless, but I don't see the point, unless some worldly change is accomplished. It seems these people exist just because they do. We all do yes, but I can't imagine living out my days with neither offspring or impact upon this planet.
If I had not child nor worldly deed, to whom or what do I live and die for?
All the work,challenges,pain, joy, possessions, and experiences only existed for me, and once I go, they go too. Surely once one's life is extinguished that perspective of the world dies with it. Why work so hard for temporary happiness in a life so short, the universe does not know it happened?
Because you do exist and this life you have is all you get. So living happily for as long as you can, for what ever reasons, seems like a reasonable answer to the meaning of life.
In many ways, I already have an easier life than many. I have an offspring and everything I am and do will be passed along. In essence, I will still be alive even after death. Unless I create something beneficial to the world or destroy something harmful to it, I will never triumph having a child.
Why do you?