"It seems....acting as if you are insulted is just.. ?sometimes? used as an impolite way of conceding. "
No, that's just something people assume so that they don't have to face the possibility that they might actually have offended somebody. If somebody appears offended by your words, they probably are. I advise you to reflect on why that might be.
As for actually dishing out insults, it appears to me that they arise in the following situations:
1) The insulter considers your point of view offensive, such as in the abortion debate.
2) The insulter feels you are being narrow-minded and not considering other perspectives (or not acknowledging the limitations of your own).
3) The insulter is frustrated because you aren't understanding their argument.
4) The insulter is frustrated because they do not understand your argument. Perhaps they think you are being intentionally confusing.
5) The insulter is trying to win by ad hominem. That is, they think if they say enough bad things about you, it will invalidate your argument. This is used primarily by politicians and Mike. This is why I don't respond to him or even read his posts. (Here's a good example: http://www.astronomy.net/forums/blackholes/messages/5689.shtml)
I'm sure there are more, but those are the first that come to mind. Whenever I insult somebody on this board, it's usually because of #2. It probably isn't justified, but sometimes I can't help myself. I apologize if I have said anything insulting to you, but if I have, consider the context in which it occurred.