Why are you trying to tear down other people's faith?

When will you hear it?
This response will usually come late in a discussion, or as an interpolation in an existing discussion between atheists. It's less of a counter-apologetic and more of an attempt to derail the conversation to the motives an atheist may have for arguing against gods and religion. This response is a good sign that your interlocutor thinks your words could be damning to their faith, or the faith of others.

A response
That need not be the goal of the atheist. Sometimes, explaining how we can live full, productive, happy lives without faith is enough to shake the faith of some. We can't really do anything about that, except stay silent, and that's certainly the option some would like us to take. To do so would present an egregious double standard. Christians do not shy away from explaining their faith for fear to damage the faith of a Jain, in fact they are encouraged to.

The salient point here is that most likely an atheist does not consider religious faith a benefit. In fact, it is the cornerstone of dogmatism, which is the source of most of the problems stemming from religion. Whether it is the immediate goal or not, people losing their faith in superstition is seen as a Good Thing.

Possible rebuttals
The theist might interject that people find great comfort in their faith. That may be true, however it is not always relevant. A drunk will find comfort in a bottle of whiskey and a drug addict in a syringe full of heroin, yet there are good reasons to help both these people to overcome these addictions. In fact, the original question can be easily paralleled to "Why are you such a buzzkill?" uttered by a drunk from whom you take the bottle away, and for much the same reasons. You could say that if faith can be broken through rational argument and evidence, then it was likely misplaced to begin with.

You can point to the fact that Denmark, a relatively secular country, is as far as psychologists can tell the happiest place on Earth. That gives evidence that religion is not needed for happiness. You can point to the harm religion does, for example pressurising partners in abusive marriages to stay together because of Christian opposition to divorce. You can point out that atheist websites focus attacks on fundamentalist Christians while liberal Christians who do little harm are left alone.