Why do you have to believe in god to have purpose in your life?

What's so great about god that believing in him would give someones life purpose, and take purpose away from someones life just for not believing in him? I have purpose, and fulfillment in my life without the aid of Jesus or god, but religious people say this is impossible because I have no god.......I guess I don't understand why god gives purpose to someones life. I mean people say god makes them do good things, and that gives there lives purpose......but why do you need god to do something good in the first place?