Having a clear rotation and pecking order in the regular season tends to lead to a more harmonious locker room. Winning does even more for a team's morale, but in an 82 game season you will always have some down periods. As President of Basketball Operations Brad Stevens has prioritized not adding players to the end of the bench who have the potential to whine about playing time.
With all due respect to the end of our bench the past two season it's consisted mostly of scrubs, journeymen, vets on their last legs, and G-Leaguers. Due to that, there hasn't really been much complaining at all about playing time.