Why do people make false accusations in cases of domestic violence?

It makes me very angry when this happens. My step father had a girlfriend who felt the need to do this because she had found a new boyfriend and was just being spiteful. I went to court and defended my step father and all charges were dropped. Her own son could not admit that anything had happened. Sometimes people need to admit their own guilt and own up to responsibilities. Your right, no one wins in these cases.