God created us for one ultimate purpose: to share His Love with us, that we might partake of the very life and love that exists in the Trinity. Happinesss is synomous with love. One cannot be happy and not have love. And also, one cannot have love and not have happiness. In loving so much that we even suffer for those who infinitely insult us is the greatest manifestation of God’s glory. For example, why is Star Wars so great? Because of light sabers and spaceships, or because a man started out good, became evil and was redeemed. That God is not only able to have mercy on us but also restore to us the very life and love within him into the depths of our soul is the essence of God’s glory.
“God is Love.” It says this twice in St. John’s first Catholic Epistle. I think he was trying to emphasize something here. God doesn’t need us, but He WANTS us, He wants to share infinite love with us and give us an opportunity to love even as he loves, even if only in a finite sense.
But the modern world cannot comprehend this, because it’s conception of happiness usually never goes beyond physical pleasures, possessions, and egotistical and superficial accomplishments. The modern world cares not even for natural love, let alone supernatural. So then, until the atheist desires to receive and give true intimate love, priceless love, he will never understand why one would even want God to exist.