[drewisajaguar]

One thing that I think plays a role in why a believer would engage in this kind of conversation/debate/argument with a non-believer is that many religions instruct their followers to convert the unconverted and tell them that it is their right and responsibility to do so. As to why it usually ends up in an argument, there could be many explanations.

I've engaged in this kind of conversation with my mother on several occasions (she's a Christian and I'm an atheist) and she's told me that she gets defensive because she feels like I'm attacking her beliefs. I do my best to talk with as much respect as I can, but I tend to find myself questioning every argument she comes up with, so I would assume that's why she ends up feeling that way. I should also note that, as a Christian, she questions every argument I have as well, but I don't find myself becoming defensive, so take that to mean whatever you want it to mean. This example doesn't go for every situation of course, but it is one explanation I've encountered.

For myself, I must repeat QuantumModulus in saying that religion bothers me personally because one's religious beliefs tend to have an influence over the decisions they make, especially in regards to things such as politics. Many politicians cite their religious beliefs for the reasoning behind their support or opposition of a bill or law. When enough politicians who have the same religion come together, that religion tends to have a large influence over the bills that are passed and the laws that are established.

I also must repeat The Man And XX Master in saying that I find it rather annoying and pointless when one argues from a non-scientific standpoint as I have found most religious people often do.