In a lot of press conferences, Smart literally would say "the team needs me, this team will succeed when I'm out there hustling", etc. A lot of "I" in his responses. And look, some of that was fine in 2022 when he was the DPOY and playing well for the team. The problem was, late in games, he'd be out there over White/Brogdon and teams would leave Smart open and say, "sure, go ahead and keep chucking". And he would and he'd keep predictably missing.
Remember 2022 ECF Game 7 on the road? The final 3 minutes where we nearly blew a double-digit lead and probably would have if Butler hit the 3. Smart bricked a ton of shots before then and also took some bad, early shots instead of letting the team run the offense and search for a better shot.
And do people also remember when before playoff games he'd almost be "coaching" Mazzulla on the sidelines, and once in a game he literally substituted himself into a game without Mazz's approval/knowledge beforehand. I know Mazzulla gets a lot of criticism but idk if it's a good look to basically publicly undermine a HC
He definitely grew a bit of an ego in his final few seasons here.
Again, I do love and miss Smart, but the reality is he's best off as a 6th man on a team, not a starting guard. And with him gone, it definitely seems to have given the keys more to the Jays and allowed them to be real leaders and not have any question's about that since Smart was technically the "longest tenured Celtics" before 2023.