I haven't slipped away, but I have a few ideas as to why.
I went to a ministry convention and I believe the facts were 80% of regular church goers stop going to church and lose relevance once they hit college. Alot of kids go to church and are really just living out their parent's faith, and haven't really taken it up on their own. Although some have. The Devil throws temptations to those strongest in God, cause they're the ones who threat him the most. Therefore several obstacles can wear people down, especially teens who already have struggles with growing, Highschool, and what not. With all the obstacles we may start to think why is this happening to us, if God really existed and was really in control why would he let these things happen. Which is a horrible thought in my opinion, because God is in control and is completely capable, but he has given us all our free will to chose to do right from wrong, and the devil will tempt us. These temptations though if we overcome them will strengthen our faith and teach us something God has been trying to reveal to us. All of the temptation and God's control in matters is shown alot in the book of Job. In Job you can see both sides of the story.
Evil desires and wanting independence often drives people away from God. Christianity is alot about submitting your life to God and taking his path, and letting you lead him. Which turns out being the best thing for you and making the journey easier and more enjoyable, but people often see that as someone making all teh decisions for them and we especially teens want our independence and to be able to make choices for our own. Alot of teens start wanting to have sex before marriage, alot of christians even, have lost sight of God's will and have decided to ignore it and live by just certain rules.
There are soo many things that can pull you away from God. Alot of people just don't have deep enough roots in their faith and they go out to witness, or hang with their friends in general.. but they aren't the ones rubbing off on the bad role models, they're the ones being molded into the wrong things.
The Parable of the Sower Matthew 13 is a parable of different ways people are torn away from Christ.