I haven't left yet but I certainly will.
The reason? Well... everything. The whole JW doctrine/theology/cosmology for me is as alive as a phantom of a zombie of an imaginary person: you have to limit yourself (i.e. not be 100% who you are) in order to be fit for an utopian Paradise, based on doctrines that are unreasonable and badly justified, which are defended by a bunch of people whose authority comes from a false date and false prophecy, which are based on a book of myths that claims to be written by an imaginary God.
But if you ask for which doctrine made me doubt for the first time... I guess that mandatory preaching was the thing that has bothered me since I can remember. Of course at the beginning I couldn't think of an argument to debunk that doctrine, but it planted some "anger" seeds in me that little by little made me end up thinking "fuck off" and started feeling rebel about "shunning" policy, "worldly people" and everything that came next.