This is interesting so I thought I'd just add my .02. I agree with the competitiveness thing. I think the only times guys compete with each other is if they both want the same thing, a car, a house, especially a girl. But if you remove that element, I think guys usually are more likely to get along than women. Women compete over butt size, age, beauty, etc... stuff guys generally don't care about.
But also, I think guys expect less from each other than women do. We don't need constant friendship affirmations, and it takes a lot for us to really insult each other. Whereas it seems that with women, if something can be taken wrong, it certainly will be, a remark, a look, going too long without talking, that kind of thing. Guys don't give a shit about that. I've got a friend from high school whom I see maybe once a year for the last seven years or more. Yet I still consider him one of my best friends.
This is all a based on my own observations of course. There are a lot of women who don't fall into this category at all. But I have to say that I believe most do.