His name is apparently Jack Napier. He became the Joker because he watched his wife, Mary, die of a gunshot to the head while she was holding their unborn child in her arms.
Henceforth he sought revenge on all those responsible for the crimes perpetrated against him and his family as Batman’s arch nemesis, The Joker. And because no one knows who would adopt such a persona unless they had undergone a severe mental breakdown.”
Anyone who has gone through trauma may well not want to deal with it any longer and give up on life completely by becoming comically crazy in order to escape its pain.