Christian brothers and sisters: Why does sin have such a hardening effect on ones heart and personality?

Anyone who has ever observed those immersed in gross immorality, especially over the long term, could easily see the hardening effect it has on ones personality and heart. Such people usually talk with a monotone voice with very little personality in it, and are very callused ,crude ,bold and insensitive. Why exactly does sin have such an effect ? Does the bible give an answer ? Or is this simply the result of sin that separate us from God who is the life giver and sustainer ?