There are many ways to answer this but the simplest one is to say that bad things happen because we live in a fallen world. The rain falls on the just and the unjust. Ever since sin entered the human race, we have lived in “paradise lost.” Sometimes we get the wrong idea about things. We think, “If I serve the Lord, I won’t get cancer. I won’t lose my job. My kids will do well. And my marriage will thrive.” But the Bible nowhere promises that his children will be exempt from the common suffering of the world. We suffer because we are human and we live in a world marred by sin. Sometimes we suffer because of the sins of others, e. g. when we are hit by a drunk driver. Sometimes we suffer because of our own foolish choices. Sometimes we suffer simply because the world itself is out of whack. That’s the only real explanation for tsunamis, tornadoes, cancer, and all the other catastrophes that afflict us. There will no release from these things until sin itself is removed from the universe. But that day is coming. Romans 8:22-23 speaks of the whole creation groaning, waiting for redemption. Today we suffer because of the fall. But in the world to come, when sin is gone, cancer will be gone too. Death will be no more. Between now and then, we walk by faith, trusting God even when life tumbles in.
See Be Still and Know.