After LeBron James left for the Los Angeles Lakers, the Boston Celtics were in a tier of their own in the East. There was not a single team that should have been able to contend with them, but that is not how they wanted it.
The Celtics have embraced and flourished with adversity throughout this entire rebuild, and they are always at their best when things appear to be getting harder. With an incredibly easy Eastern Conference, no team would have pushed the Celtics to their best, and that would hurt their chances later on, and make them more susceptible to an upset in the playoffs.