It's human nature, not sure which part, maybe the part that refuses to believe they spent money on the wrong thing.
I've seen this for as long as I can remember, like you - I bet there were NES fans dissing your MSX, and ST fans dissing your Amiga, and... Well it's just the way it is. I defended my ZX Spectrum against the C64 right upto the point that I got a C64, then I defended the Atari ST against the Amiga, even though the Amiga was clearly a better machine. It goes on, and on, and will never stop. People do it with phones, cars, sports teams etc as people have already said - human nature.
I decided a while ago to stop being like that - once I aquired an iMac and iPod - I doubted these things, being a PC user primarily, but damn do I like them now... but owning an apple product puts me right in the firing line.
Things might change though, some people like me own all the options, PC, Mac, Xbox, Wii, PS3, PSP, DS... so there's a more educated opinion I think - I mean I was totally wrong about Mac's, so I go by experience now, if I can help it. When you have that informed opinion, well it's absolutely mindnumbingly annoying when someone ignores it - I love to challenge people who diss one console etc without even trying it. Some people can't afford their own opinion, those trolls are the ones you have to filter out.