Theodicy, or how we explain the existence in a just and loving God in the face of a world that seems to mock any notions of fairness or justice of any kind.
The most compelling reason I've ever heard is neatly expressed by Rabbi Harold Kushner in the book "Why Bad Things Happen To Good People". He simply notes that God could simply compel our worship-you know, being all powerful and all. But He does not- he allows us to choose to believe or not, so that the belief will be meaningful. The freedom to choose, of course, allows the freedom, in turn, to choose otherwise, and the existence of evildoers.
Boston plays the mighty Redlegs again in the Queen City today, and the NBA Finals is on at 9pm tomorrow.
Happy Father's Day to everyone tomorrow.