Group Politics--UGGH!

Why is it that people can be so difficult to work with? Especially in a group setting?

The past few years have been a great learning experience. In our Home Owners' Association meetings, I seen meetings where people have been ready to start a fist fight over seemingly tiny things. I've seen people who get upset at the drop of a hat, and bring their own hat. They seem primed for a fight, or assume that anything not done their way is done intentionally to get to them.

Now I've inherited leadership of a long-standing homeschool support group. I volunteered to help a fairly large group of people lead the group and ended up being the lone leader standing as people quit or disappeared for months at a time. I don't mind doing the work of organizing and such, but the bickering and back-biting, the people who assume the worst of other people, the people who want to constantly complain about the way I do things but don't want to help lead, either, gets old fast. I've tried accomodating them, trying to use their ideas and include them, but it seems to have back-fired and left me with more of them mad at each other.

Guess it's time to let some of them just leave the group and move on. But why is it that people who came together for the same purpose initially can't just get along? Why can't they be civil and work through their problems? Why?