There are many reasons for me but probably the most important is that I find theists to often buy into irrational stories that serve as justification for them to do some of the most despicable acts the world has ever known.

Even when their beliefs don't lead them to actually kill others for no good reason, they can lead them to deny rights to gays, be indifferent to long term environmental planning because some are convinced that the world is ending soon anyway, block stem cell research which could otherwise extend my life and the lives of loved ones someday, deny or impede access to birth control which creates unwanted babies and spreads deadly diseases.

Do I need to go on? Because I probably would if I thought it might convince someone to reconsider their unfounded beliefs which lead to what I believe to be so many incredibly cruel actions.