"If I have not love, I am nothing." Kinda says it all, doesn't it? I believe Whoever or Whatever created this apparent reality acted out of Love and made the attainment of unconditional Love within each of us our ultimate goal. Only when this goal is met are we fit to reside within the Creator and feel fulfilled there. Not one thing this world may offer us should tempt us to fail Love... but alas, most of us do.
The Buddha said we are unhappy because of our desires. We continue to want in the material sense, in spite of all we may have materially. So we are perpetually unhappy in this world right to the day we die, still wanting. And due to our death in such state our Creator allows our reincarnation here since That is what we wanted; in spite of all the pain, sorrow and disappointments we will yet again live through.