"If God is good, why do bad things happen?"
This is a question we all have to grapple with at one point or another. In fact, the Book of Job was written specifically to wrestle with this question. Job never really answers the question, except to point out God's presence with us in our trouble.
At the end of his book What Shall I Say?: Evil, Suffering, and the Crisis of Faith, preaching expert Thomas Long discusses The Parable of the Wheat and the Weeds in Matthew 13:24-30. I have found this parable particularly comforting when it comes to the question of "why do bad things happen?" In the story Jesus tells, we never hear where the enemy came from or how the enemy got the weeds to plant in the first place. All we know from the Parable is that evil exists in the world, but more important we learn that God, goodness, will win out in the end.
There are so many things we do not and cannot know in this life. We always have the comfort, though, that God is present with us even when we cannot see Him and that even when it doesn't seem like, good will have the final say when this world comes to an end.