Because as sinners we were in slavery to the prince of darkness, in bondage to him, most d words refer to satan, depression dispondency ect anyone who has experienced these d word knows just how dark they are, but once we are saved, translated into the kingdom of God, we experience a change, a lightness, revelation that God, Jesus is our salvation, Jesus is light and in him there is no darkness at all- that says it all for me, why would you choose the dark? Darkness speaks of an absence of light,warmth and joy, Gods kingdom of light has all these qualities. Why would you go back, sometimes we need reminding of just where we have been.Gods way is a better way, we were given a free will and the choices we make have outcomes, we know right from wrong, good from bad, so the choices we make govern our outcomes, satans rule or Gods rule, both goverments dominion or kingdom, which ever you choose you will be subject to the outcomes