Why do Jews believe bad things happen?

Christian answer: They probably believe bad things happen for the same reason most other religion does. And this reason is because of what is usually called sin. There is a sort of master of all evil and his name is Satan or the Devil. He is responsible, in all three of the largest religious beliefs on earth (Christianity, Judaism, and Islam), for bringing sin into the world because he caused the first humans to disobey God and commit the first sin. And because of this act, sin is inherit in every human being and is often called man's sin nature.