Why do some Christians seem to be personally affronted when they become aware of an atheist?

Most often it will end with a lecture on salvation. I don't want to hear about how I'm going to burn in eternal hellfire because I don't believe in God-that's really nobody's business but my own. I don't understand why I am confronted about my beliefs (people can do this kindly or angrily, both ways are irritating). I don't care about people's religious preferences, and I certainly wouldn't belittle someone who believed differently than I do, I guess I'm just wondering where all the anger is coming from?