Getting Used to the Dark

I have a child that just couldn't process the thoughts they were having.....How can someone who once followed God passionately not even give Him a second thought?  Why has their life changed so much they just seem selfish now?  Why is it the closer they get to God, the further away they seem?
I don't know the theological reason, other than SIN.
  Here's what I answered....
We have a free will.  A choice to make.  Every day we choose who we will serve.  The day we stop choosing and just go with our heart, which is deceitful above all things, is the day we take our first step into the darkness.  It becomes easier and easier to not make the choice anymore.
You know when you go into a dark room and eventually your eyes adjust to the dark?  You can now see what's right in front of you?  That's how it is when we start living more in the dark that in HIS light.  We only can see ourselves.  We only think about OUR next step and forget how wonderful and good it was to be in the light.  We think because we can see something, we're ok.  We're missing out on God and focusing on ourselves.  Relationships suffer...and we don't even realize it.  It hurts watch this happen.  Sometimes it happens fast, sometimes it takes 15 years.
What would you say?  I welcome advice and feedback.