here's my take: It's not the same for everyone, there are several camps.
1. The "It just works camp" - these are people that don't demand a lot from their electronics, but they want it to do it well. In this category are my sisters, who both got mac laptops after they got tired of me fixing their dells. They don't care about file structures or chipsets or Unix, they just want it to work and now that they've had that experience, they have a prejudice toward apple products.
2. The "I've been around the block camp" - People that are in stark contrast to the one above, this group has used windows, os x, and linux. They choose Apple b/c their values align with those of Apple. Those who have different values are right now on their respective windows or linux forums. It's like people that always drive Hondas or BMWs b/c they know what they are getting and they like it and they have experience with Toyotas and Fords and the like.
3. The "I think this makes me hip camp" - Although this probably bothers most of us on these forums, I'm sure there is a significant number of people who have turned into Apple fanboys b/c it buys them cool points. Macbooks and iPods stand out as cool products and if you have one, then you too are "cool." They get a thrill out of telling people their Dells suck. I've got a little cousin who falls in this camp. Two weeks after he got his iMac he was talking down to every PC user he met, but he is 12 so I'll cut him a little slack.
4. The "Been here since the Apple II camp" - People that have been with the company and it's products so long that nothing else matters anymore.
is it possible to be a 1.5?... haha. thats me