Take time to read this. Maybe it is to the point of some of what you are asking. Its in part about fighting over ideas and why we need to do it.
http://online.wsj.com/article/SB121736628530894671.html?mod=opinion_journal_federation
What sort of opinions are you talking about tho...? Awful big percent of what is posted is an opinion. Out of competing opinions, well or badly stated we see some sense usually.
Now of course, sometimes people like to just butt in in and say something derisive. For example: One time i offered the opinion that a marlin 336 is probably more accurate than an M14. That was based on my personal experience. Now, "s.....k" thought it suitable to respond by saying quote, "HAHAHAHA" , do i really believe that, and how old am I. Well, that was good for a fight until he just got too offensive.
Be that as it may, if I can paraphrase your question a bit, are you wanting to know why we cant just keep our opinions about the personal qualities of others to ourselves?
I'd say its two reasons, basically.
One is, some people of primitive brain think that attacking the other person is a way to negate their ideas.
The other is, some people are just plain jerks.
But hey, its kind of the free market here, if you dont like how it is dont come. You are not going to change anyone.