There isn't much that we can be sure of in this world, but one thing that we have come to realize is that bad things happen.
We had to bury my 17-year-old sister halfway through her senior year. Our friends lost their four-year-old to cancer. My best friend lost his dad when he was just in 7th grade. A 16-year-old from my hometown was just killed in a car accident. Tragedies hurt a little more when they hit close to home, and "why" is the question that is front and center in our minds.
Biblically, why do bad things happen?
First of all, God didn't create evil. Genesis 1:31 says, "God saw all that he had made, and it was very good." When God created the world, He created free will. With free will, He created the potential for good, but also the potential for bad.
Eve made the choice for all of mankind back in the Garden of Eden. Before she chose to go against God's word, sin was not in the world and evil was not an inherent part of human nature. God didn't want us to just be His puppets, so He gave us the option to choose our own actions--He gave us the option to choose whether or not we want to love Him as He has loved us. Ever since this first act of sin, we have to be actively choosing between what is good and what is evil.
Author Lee Strobel said it well when he said: "In other words: look at your hand. You can choose to use that hand to hold a gun and shoot someone, or you can use it to feed hungry people. It’s your choice. But it’s unfair to shoot someone and then blame God for the existence of evil and suffering. Like that old cartoon said: 'We have seen the enemy, and he is us.'"
Just because God is not the one who causes bad things to happen doesn't mean that He can't use them for good. "And we know that in all things God works for the good of those who love him, who have been called according to his purpose." Romans 8:28
God chose to take the worst thing that has happened in all of history, Jesus Christ (who was perfect and sinless, by the way) being brutally murdered through crucifixion, and turn it into something good. If Jesus hadn't died on that cross, we would have no hope; there would be nothing to save us from our own sins. So, if God can take His own son being killed and turn it into the best thing to ever happen, then He is more than qualified to turn our suffering into something good as long as we are living for Him.
"In all this you greatly rejoice, though now for a little while you may have had to suffer grief in all kinds of trials. These have come so that the proven genuineness of your faith--of greater worth than gold, which perishes even though refined by fire--may result in praise, glory, and honor when Jesus Christ is revealed." 1 Peter 1:6-7
As humans, we seek for concrete answers. Do they always help? No. Just because we have some answers as to why suffering happens does not mean that our human brains can understand it any better/cope any easier.
To put it frankly, life sucks sometimes. We know that bad things are going to come our way, Jesus told us flat out in John 16:33 "In this world you will have trouble." The hope that we can rest in when terrible things happen is hope that, somehow, God will turn our sorrows into joy.“You intended to harm me, but God intended it for good to accomplish what is now being done, the saving of many lives.” Genesis 50:20
Read to the end of the book, suffering is no more. Goodness wins out over evil.