On the contrary, from my experience it isn't.

Sorry, I could not resist the opportunity. But seriously, I don't often see people disagreeing for the sake of disagreeing. More often, they'll point out different aspects, or their own perspective on a topic. To be honest, support and affirmation are perhaps a bit rarer than they should be, but I've rarely perceived disagreement to be hostile, as opposed to misunderstanding, or legitimate and resolvable via further discussion.

More datapoints, anyone?