They're on the defensive, because there so far isnt a logically sound platform for an augment on god(s). They also tend to be on the affirmative, and have the burden of proof.
Most atheists I've come across also with the burden of proof because they are so aggressively affirmative when there so far isn't a logically sound platform for an argument on gods. By-the-book theists tend to be on the defensive because they try to defend, as was first discussed in this thread, a several thousand year old hash of translations and edits and numerous things you can't take literally.
Beside, that, I find the stance of letting everyone believe what they want to believe without examination is dangerous.
Contesting belief is vital to weed out dangerous ones and erroneous ones is a good thing.
Granted I agree there are lines to cross where a person's beliefs should be questioned, generally the insane asylums and prisons hold those individuals from society. What justifies that as your personal duty? How many "dangerous" theists have aggressive atheists saved the world from by converting? Generally I find when under pressure from atheists, many theists snap to extremes, either comically hostile atheist convert or now dangerously irrational theist. You're going to make people more sane by undermining their faiths and challenging them? How does that even sound like a good idea in the first place? I'd like to see you try to walk up to an irrational theist, say, a suicide bomber, and make him come to your realizations that everything he knows is wrong without him blowing everything to fuck up in response.
I have never in my life seen an atheist trying to convert someone do any good. Ever. In fact, I've only seen bad things come of it.
Atheists who try to convert people are assholes, especially if what they believe is true. This is hard to explain, but let me try. It's morally fucked up. I know this has been said around here at least once before. Now imagine we are indeed only dust in the wind. What is accomplished by forcing this on people? There's nothing they can do about it. They were happily asleep until you went about and woke them up. You ever wonder why some people gravitate towards religion and spirituality? It's because it's for them. It suits them. Some people prefer to function under what they find to be complete awareness about life. It suits them as it suits you. They also have a tendency to gravitate towards atheism completely on their own. There's no need to go about and force these things on people.
So why? Why do you find it a life mission to undermine people? You sure as hell aren't making the world safer from "irrational" theists by undermining their faith and beliefs and you sure as hell aren't doing those you "enlighten" any favors.
Atheists need to keep to themselves, never mind the fact their conclusions are as unfounded as the people's they attack. Theists need to keep to themselves, never mind their conclusions being unfounded. They don't mix well. Ever. Nothing good will ever come of them trying to force anything on the other.