In some ways, the WTS did me a big favour. Before I became a JW back in 1982, my upbringing was pretty a-religious. My folks were non-practising Catholics and I became more committed to Catholicism when I went to University in 1981. I didn't really think much about the issues and it didn't have much impact on my daily life. Becoming a JW meant I had to evaluate my religious beliefs and because I hadn't done much research, didn't know how to and wasn't very switched on, it wasn't difficult to turn me away. It was however extremely damaging to my education, family life; friends and career. The WTS did a pretty good job of trashing other organised religions so when I finally got booted out in 1986/87 I think..I was a whole lot better informed and in some ways innoculated against organised religion. I knew what to ask and where to look.
I think it must be very difficult for those brought up as witnesses and programmed from birth to turn away.Thankfully many do although I sometimes wonder how many stay and suffer in silence or blank out their doubts. Maintaining two frameworks could be very damaging psychologically I would think
Following my velocitous extramuralisation (for Blackadder fans) the process of evaulating the WTS began (because at that time I thought it was "The Truth") and I have to say I learned a great deal over many years, not just about the WTS but about other belief systems.
I suspect the reasons people leave the faith of their youth are the same for many organised religions. Most look pretty similar to me. They are simply not relevant to the followers who leave them or there is some other kind of disconnect which occurs in the persons life; a bereavement; poor treatment; a lingering doubt that grows; questions that cannot be answered; a decay of certainty etc. Either the adherent never believed in the first place or there is insufficient foundation inherent in the belief system to resist critical examination and a need to rely on "faith" when the answers run out.