June 2024 felt like the beginning of a dynasty. The Boston Celtics had just dismantled the Dallas Mavericks in five games to claim their record 18th NBA championship. Jayson Tatum was ascending. Jaylen Brown had his Finals MVP. Joe Mazzulla, the young coach everyone underestimated, had quietly built one of the most dominant teams in the league. The Celtics weren't just champions — they looked like the team of the era. One year later, they were watching the Knicks celebrate in the second round. So what happened? The Run-Up: Four Years of Elite Ball To understand 2025, you have to zoom out. The Celtics have been a genuinely great team for four consecutive seasons. That's not a small thing in a league built on parity. Here's the arc: 2021–22 (51–31): The resurrection. Ime Udoka's team came out of nowhere, ran through Brooklyn, Milwaukee, and Miami, and pushed Golden State to six games in the Finals before losing. 2022–23 (57–25): The near-miss.…