As a rule, those who reject religion do so because they have allowed their intellect to consider both the evidence of religion and science. Evaluating and comparing the mythology of religion with the methodology of science never ends well for religion, unless one first shuts ones mind to reason.
This process of rational evaluation is ultimately behind the philosophy of all non believers.
Thus whenever one of the faithful ask "why are you an atheist?" One is always inevitably accused of "attacking" religion because, in sharing the rational thought processes that lead one away from religion, the listener is forced to consider the rational arguments for abandoning the codified superstitions that make up his or her religious beliefs.
This insecurity about the veracity of their religion stems from their twin subconscious fears of is Death really the end and is there no force or power in the universe to look after me when I'm out of my depth. In simple terms, they are seeking the reassurance that they had from their parents or perhaps grandparents when they were children in a big scary world, even though they are now adults.
Thus, in asking a thinking person about their thought processes, they open the door to a rational and intellectual examination of their beliefs, which is the last thing they want to do. Additionally, as the atheist views the world through the lens of rational thought, the religiously inclined are forced to recognize that their world view, being based entirely upon faith, rather than reason, tend to feel the atheist is "judging them intellectually" and in doing so "finding them to be stupid".
Of course it's nonsense, the fear of being judged and found wanting is a religious hang-up, they bring that psychological baggage to the conversation and the atheist who is simply giving an honest answer to a question he or she was asked gets accused (erroneously) of attacking the beliefs of the questioner.
Thus, the only advice I offer to my fellow atheists when it comes to answering such a question is to begin by clearly saying, that what you're going to say is how you see things, but you accept that other people may see things differently, and whilst you respect their rights to their views, you personally have a different perspective.
Even starting like that, only one in ten religious people (if you're lucky) will accept that you're not attacking them, but let's face it, that's a big improvement over what usually happens.