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 steer the conversation away from the evidence for or against positive god claims and toward other personal motives an atheist may have for rejecting such claims. This response is a good sign that your interlocutor finds your words threatening to his or her faith, or to the faith of others.
Tearing down the faith of another person need not be the goal of the atheist. Sometimes, explaining why we do not accept positive god claims, pointing out the harm that can come from belief without evidence, or explaining how we can live full, productive, happy lives without faith is enough to shake the faith of some. We are not responsible for the doubts of the faithful, and we can't be expected to refrain from reasonable discussion on the topic merely because some people want us to. To do so would present an egregious double standard. Christians do not shy away from explaining their faith for fear of damaging the faith of a Jain or a Hindu or a Muslim. In fact, explaining, arguing for, and propagating faith is generally encouraged within Christianity.
The salient point here is that an atheist most likely does not consider religious faith a benefit. In fact, many atheists view faith as the cornerstone of dogmatism, which is the source of most problems stemming from religion. Most atheists view the loss of faith in superstition as a positive outcome, so atheists should feel no compulsion to refrain from discussion of god claims and their effects on society.
The theist might interject that people find great comfort in their faith. Although that may be true, such comfort does not imply that faith produces true beliefs or productive behaviors. 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. A reasonable response is 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 psychologists have named Denmark, a relatively secular country, as the happiest place on Earth . This provides evidence that religion is unnecessary for happiness. You can point to the harm caused by religion. For example, a partner in an abusive marriage often is pressured to stay with his or her abuser because of Christian opposition to divorce. You can also point out that atheist websites focus attacks primarily on fundamentalist Christians instead of on liberal Christians, who do relatively little harm.