Perhaps the question could have been phrased more like why there is such animosity between atheists and theists or do you think that is the case?
There is inevitably animosity between people who want to tell others how they should behave, and those upon whom they seek to impose those rules.
If a Muslim chooses not to eat pork, I couldn't care less. But if he tries to stop
me from eating pork, we are going to have a problem.
If a Christian chooses not to buy alcohol
on Sunday, I am perfectly disinterested in his decision. But if he tries to make it illegal for a publican, who wants to sell beer on a Sunday, to sell me a beer on Sunday, we are going to have a problem.
There is animosity between the religious and the irreligious if, as, and when one group tries to impose their wishes upon the other group.
Some cases are less clear cut. If a group of local Muslims want their Muezzin to loudly call them to prayer, and I want to enjoy peace and quiet, then are they imposing on me by making a racket; Or am I imposing on them by demanding that they remain silent?
What if the noise is made by Christians ringing church bells on a Sunday, or for a wedding, or a funeral?
This is, of course, not a religious question at all - what if the noise is an atheist listening to his death metal records turned up to eleven, while I am trying to enjoy a quiet cup of tea next door?
Still, the atheist music lover at least doesn't have the brass face to pretend that it's not his problem, because his imaginary friend said it was not just acceptable, but mandatory, to crank up the decibels whether his neighbours like it or not.
The reasons for animosity between theist and atheist are fundamentally the same as the reasons for animosity between people of any belief, or none - people impose on each other, and those who are imposed upon often don't like it.
When one side of a dispute claims divine right, the dispute can no longer be as easily settled by compromise, because religion is by definition uncompromising.
That's why disputes between theists and atheists are more often heated than are disputes amongst atheists; And why disputes amongst theists are so heated that they often lead to terrorism, murder, war, genocide, and strongly worded letters to
The Times.