In a word, sin!
When God initially created the world (the cosmos)—including man—everything was perfect. But man (Adam and Eve) sinned, resulting in what is referred to as “the Fall.” Throughout history we see the results: anger, hatred, wars, disease, etc., etc.
But in His wisdom and love, God sent His Son, Jesus, to save us from our sin and its dreadful penalty. As Christians, we still live in a far less-than-perfect world, but one day, when we breathe our last, we will be instantly ushered into heaven!