People have historically hated the Celtics because we won more championships than anyone - from Russell's first in '57 thru Bird's last in '86, we won 16 titles in 30 NBA seasons with three separate groups of players. The Celtics won over half of the available titles.
Plus, you had Red Auerbach, who we love, but who, to rival fans, was just this arrogant jerk running our franchise who would do anything short of breaking the law to gain an advantage over the rest of the league. He delighted in firing up those Macanudos towards the end of wins and didn't care who despised him for it.
Celts have never really had a lot of bad guys or villians - we just had better players and Red was great at building championship groups, one after another after another. And he built the teams thru the draft and a few select trades, not by getting all the best players to come work his summer camp so they could become best friends and jump on the same team to steal a title.