Why are disputes so belligerent in online communities?

I've noticed that people get into very loud and heated disputes online, whereas that doesn't seem to happen in local communities. Disputes in local communities tend to be less frequent, less belligerent, and last for a shorter time -- even when some people end up hating each other and refusing to have anything to do with each other in the end. Why is that? Also, why do people who are closest with each other (whether close friends, dating, or married) seem to agree more on hot-button issues? Are people are more willing to reject a stranger's arguments than those of a friend? Is that an error?