I asked my jw the following as well, "Why do they leave if it's the truth?" he says it is because they don't want live by rules and have someone in authority over them. Basically they want to go out and become sinners, sex, drugs and rock & roll kind of lifestyle. They think they can live their life however they want.
Yeah I know, I'm sitting here shaking my head too.
Ask a jw if they are a sinner, listen to them spin that one!