Christians believe that the simple answer to that question is: sin. But sometimes we are a bit fuzzy on how to explain or define sin. And especially in our culture, which stresses personal autonomy and independence, the Christian doctrine of sin, and the broader Christian worldview of which that doctrine is a part, is very unpopular.
It is harder to grasp and treasure the riches of Christ's redemption without a firm grasp of sin. So for a primer on sin that will help you understand and explain what is wrong with the world, I'd recommend this essay by Cornelius Plantinga called Sin: Not the Way It's Supposed to Be (you can also view it as a 23-page PDF file).
It is basically a condensed version of Plantinga's book by the same title. But this essay gets to the heart of the book's content in a shorter and more accessible form. It may seem morbid or unnecessary to read 23 pages on what sin is, but I found this essay well worth the investment of time.