Good question.
I left the JW organisation around 10 years ago, but consider myself as still agnostic, therefore I havent given up hope on if god exists or not.
What made me turn from being a believer to being agnostic? Answer - being unhappy. During my time as a follower I felt myself sliding into a state of being unhappy, miserable, frustrated and eventually depressed. I used to ask myself 'If this is god's organisation, then why am I feeling like this?'. Since leaving the organisation, I have developed my secular career as an engineer, met a fine lady, started a family and am much happier ten years on.
Now when I think about it, It p*sses me off when you'd hear a talk or read a Watchtower / Awake article about the Devil trying to make us feel depressed and worthless, when really it was the JW teachings and general way of life that was making me feel that way.
In addition to the above, since then I have come to learn that much of their doctrines / teachings are inaccurate and erroneous, and at the time of my entering the JW faith, I was a bit naive and gullible, and didn't really question it properly.
However, just because I have rejected the JW faith doesn't mean I have given up on the hope of there being a god up above (or below, or wherever). I need more evidence that he does exist. Can anyone prove this beyond doubt? I don't think so. Many people come in his name citing verses from the Bible, Koran, Torah etc, but no one can properly prove that he exists and we are his creation. Therefore as long as this is the case, I will remain an agnostic.