Some people, whether religious or otherwise, try to force other people into their mode of thinking. It is possible to express your beliefs and even carry on debates while maintaining a respectful attitude, although to hear some people carry on you wouldn't think so. Such bitter argument is actually counterproductive. You may have valid points but if you bring them up in a hostile and combative attitude you're not going to win anyone over with them.
Because they think your acts or deeds are worthless because they are not done through christ. It's like "I just made every human on earth orgasm simultaneously with my mind!" them :"Well I felt that but was it done through christ?" "no" "well then it sucked and was not good, you shouldn't do witch craft."
Annoying behavior is not the exclusive provenance of theists. It's a two-way street, in my experience. But tolerance of others, and especially of religious differences, is our American way. I am certain of one thing: I am as supportive of the rights of atheists to not worship as I am of theists to worship, and of both their rights to free speech.
Ugh, they want to try to forcibly spread the word and trying to persuade people that God is real..Which I think is so dumb because EVERYONE has their own opinions, and everyone has the right to pick any religion they want.
What I am seeing here is ridiculous!!!! You people are complaining on how annoying religious people are, when you don't realize how obnoxious you people are acting!!! You are talking to them like they are brainless idiots who believe in "fairy tales" and you expect them to act tame? Love one another! God made us to speak our own minds and have free will, but he didn't make us so we can spew hatred towards one another! We need learn how to be professional here and stop acting like 13 year olds!!!
I'm not sure this would be the only reason, but sometimes when we feel that someone will just not listen "to reason", it can seem very irritating. And, christians who are fully convinced may listen some to others, but don't seem to change or fluctuate from their beliefs and they just seem to stand firmly. It may appear at times, that they don't care about another person, when actually, it is quite the contrary, they do care and because they care for others and for themselves, they won't bend or change for any reason as far as swaying from the truth they have come to know and believe. I'm sure there are other reasons and some maybe not honorable possibly, but maybe they have been covered in other answers or will be. So, I just mentioned this one.