Here's a proposal - one that's suited more toward a feudal/manoral setting like D&D.
They don't get along, because there's not enough resources for all of them, so they've got to compete. A commoner can only farm so many crops, an artisan can only build so fast, and even casters have just so many spell slots per day. Too many spoons, not enough soup.
Assuming it is a fiefdom - and that doesn't always have to be the case, the resources are mostly land and labor. A minor noble inherits and defends their dukedom or earltude or whatever, which is supported by the peasants there who pay taxes or tithes for the privelige of residing and working there without being eaten by some horrible monster. That minor noble is in turn accountable to and maybe loyal to a higher noble, and on up the ladder. But sooner or later in a system like this, someone's demands will exceed what their resources can make - that's when they go out looking for more lands and laborers, by fair means or foul. Whether it's got by marriage, treaties, or conquest, territory and the fealty of the underlings on it is the medieval equivalent of 26" rims.
Commoner toil just isn't all that efficient. There are substitues for the more is better method. Picking up the loyalty of just one other noble, or a high-level brawler/caster, can make a big difference. It'll also make whoever they jilted mighty unhappy, even if all the alignments involved align. There are other resources involved. Take land with a gold mine, a castle, a friendly dragon's den, and that's much more valuable than some guy with a shovel. Knowledge is a resource, a huge one in D&D. If there's a wizard's school or mysterious library (and there always is one), the group that has access to that might not want to share, earning the ire of their neighbors. Coalition is a resource. If all the burgs except one are in on some deal, that one risks being mobbed by a troll armada (and there always is one) unless they shape up. Most of all, heroism is a resource. Whoever runs out of that might have to make some very unsavory choices to 'keep up' with their neighbors. They get edgy, confrontational, and desperate - at that point, good vs. good starts the slippery slope to good vs. evil.
