From my Catholic perspective, the vast majority of Catholics who leave do so because 1)they never really knew the faith, and 2)thus they were easily deceived by others regarding the faith, or 3)left the faith for an easy or 4)more emotionalism-filled experience, or were 5)divorced or in some other irregularity regarding the faith.
Few leave Catholicism knowing the faith well but purely because they think the doctrines of Catholicism are wrong, but 1 through 5 above will generally insist this to be the case none-the-less to rationalize or assuage guilt.
Just IMHO.