There are a lot of dynamics going on from my perspective.
The first exposure I had to Macs was via my mother when I was pretty young. She told me that they were these new, great things, and her greatest endorsement for Mac was that it wasn't necessary to know how to work a computer to use a Mac. That they were made for "dummies like me," she said. The context of this is that I was learning programming languages and tearing them apart, while she couldn't figure out how to program the VCR or reset the microwave clock.
I may be wrong, but I think that on at least some level, there is an issue of self-confidence amongst some Mac users, and that they feel that PC users think they're better/smarter than they and their fellow Mac users.
Another dynamic is the artists vs. IT admins, where a lot of ignorance on both ends combine for senseless animosity. I recall getting a Mac at a business, and the IT folks folding their arms and saying, "Well, YOU'RE on your OWN if that thing has problems. WE'RE not going to fix it, because it's not compatible with the REST of the infrastructure." I agreed, because I wasn't going to ask for their help anyway. There are a lot of IT idiots that think they're holy, but really don't know half of what they should.
Yet another dynamic is that created by Apple's marketing. They went out of their way to make IBM the evil empire that must be crushed. Then they went with "Think Different." (-ly! haha) More recently, it was the PC vs. Mac, with PC as a fail-nerd and Mac as a hip, down-to-Earth-effortlessly-cool guy. This seemed to really divide the two groups the most, in my opinion. PC users saw it (perhaps rightly-so) as an insult, and Mac users developed a sense of coolness in that I-was-into-this-band-before-they-were-mainstream snobbery.
It's really no wonder that the two groups fight so much. It's all amusing to me because I'm on both sides, so I benefit from knowing that any system or device can be superior if managed by someone that knows what they're doing. Whenever I see someone cheer Apple and taunt their competition, or vice versa, I lose a little respect for them no matter which flag they carry.
To me, arguing for PC vs. Apple is as lame as arguing over religion, race or sexual preference. It's wasted, negative energy you can never get back.