I gotta agree with Bloke. Trouble is as he says an awful lot of people cant differentiate between being disagreed with and being dismissed. For me the difference is that if I totally dismiss someones argument I am very unlikely to bother arguing with them. So really when I disagree I at least see some logic they have behind their words. Too often people who arent that good at reasoned discussion take a conversation which could help them learn and develope their views as a personal slight. Thats when the flounces start.
Its magnified by typed conversation lacking any nuance or subtlety of speech so that makes it worse, especially if you have slightly dull perception to start with.
Even allowing for that though it still happens the same, both here and in real life exchanges.