You know something, on this forum, I'm glad to be able to say that the Atheists I know of don't actually feel the need to do this at all.
I think it's a small percentage of Atheists who act like this, that unfortunately give them a negative reputation for always wanting to debate the Christian belief system because they are so aggressively vocal about it on the internet (talking outside of the forum).
the reason for this is because when someone states "i'm christian" people go "oh that's nice" or "good for you". when someone who is atheist states "i'm atheist" (just quietly like i've done) they're questioned harshly. so after a while they feel the need to go on the defense because of how many times they are attacked for not believing in a higher power.
story time kiddies: recently on fb i took a "what religion are you 'suppose' to be" and i got....atheist (which i am so no big shocker there). i actually had two people whom i thought i was friends with de-friend me. wtf? because i don't believe in a god? not once have i put anything hateful, derogatory, or a joke that was making fun of someone else's belief. so i asked them. one just now ignores me and the other one asked how long would it be before i started making fun of their belief.
that's just not me.
my one aunt and her daughter are catholic and have put derogatory jokes on fb making atheist out to be the bad guy or incompetent....so after the 15th i had to know....so i asked "why is it that you feel the need to make fun of my lack of belief when i've not once made a snide comment or a joke that was in bad taste about christians?" my aunt commented that she was a good person and that she would end up in heaven.
i was kind of confused with her response as i hadn't asked her where she would end up at, just why did she need to feel justification for being christian by making fun of someone who had a different belief system. she never did answer the question....but she did stop posting insulting jokes about atheist. (i wasn't insulted just curious as to why she just simply couldn't state "i'm proud to be christian" without insulting any belief/lack of belief).
i believe in what i believe in....just as you believe in what ever your belief is....and you know what? its ok. it doesn't change the fact that we're both still humans and that we should just be respectful and courteous, no matter what someone does or doesn't believe in.