Why do you spend so much time countering god-claims?

It’s been a common observation by many theists frequenting the Disqus forum on RELIGION and the Breaking News and Religion site that is its successor that there appear to be an awful lot of atheists commenting there. And many of them appear to be interested exclusively in a particular subset of discussion: the critiquing of religion and theistic beliefs.

Some critiques are aimed at the reasons people have for believing this or that. Others are a bit more direct, and simply say “you’re wrong, and here’s why…” There seems to have developed a culture of offense and defense: atheists trying to prove the invalidity of theistic beliefs with theists defending, and – to a lesser extent, perhaps – vice versa.

Something often heard during such exchanges is the question – sometimes offered in frustration: “What is this? Why do you spend so much time attacking my beliefs? Why do you care what I believe? What is it to you?

So here’s my question to atheists who find themselves visiting this site trying to poke holes in theistic beliefs and arguments (which includes me, by the way):

Why do you spend so much time critiquing beliefs and arguments here?

(Yes, I have my reasons as well – some good, imo, some less laudable – I’m happy to discuss them below.)