Atheists and Christians, why are you so flippant with other people's beliefs sometimes?

I think is because we are like polar opposites. Some one both side can meet in the middle and discuss politely but a larger majority just bicker over who's right and who's wrong. Many times when Christians try to tell us we must repent, they don't seem to realize that they are in fact insulting us. They are indirectly saying that the way we have lived our lives is wrong. We as atheist rebuke in kind and continue the never ending bickering.