BOSTON -- Al Horford left the Celtics in free agency once. He had no interest in doing so a second time.

That, Horford said, was the biggest reason he agreed to a two-year, $20 million extension Thursday to remain with the Celtics through the 2024-25 season.

"For me, I really did want to be a part of what we have going here," Horford said after the Celtics held a shootaround at their practice facility ahead of Friday night's game against the Miami Heat. "I felt like me coming back here last year, I felt the strides the group started to make and we've continued to make strides in the right direction.

"I feel like there's a real purpose of trying to win and the group that we have is just a really good group. So, I'm happy and we know we have a lot of work ahead. But this type of stability, it's hard to find."

Horford learned that the hard way. In 2019, after spending three seasons in Boston -- and reaching the Eastern Conference finals twice -- Horford chose to join the Philadelphia 76ers on a four-year, $109 million contract, the final year of which Horford is currently playing under. But after a disastrous first season in Philadelphia in which he proved to be an awkward fit with superstar big man Joel Embiid, Horford was salary dumped to the Oklahoma City Thunder the following offseason.

After spending a year with the Thunder -- and missing the playoffs for the first time in his career -- he was traded to the Celtics for Kemba Walker in Brad Stevens' first move as president of basketball operations.

Horford said that experience, and the joy he felt last year upon returning to the Celtics and taking part in the team's run to the NBA Finals, played a significant role in him taking less to remain in Boston than what he potentially could've received as an unrestricted free agent.

"One of the things is, more than anything, you want to be in a place where you have a chance to contend, to win," Horford said. "A place that's, for me, more than basketball. Boston, I feel a real connection here with the people and with what the Celtics are all about, and it's something that just really excited me. And the fact that I get that opportunity and Brad believing in me and bringing me here, it's something that's special, and I'm really grateful for the opportunity."

Horford, 36, is signed through what will be his 39th birthday. In his 16th NBA season, Horford said he wasn't always sure he would have this sort of longevity. But as he has aged, he has become an excellent long-range shooter -- he's hitting nearly 50% of 3-pointers this season -- a change he credited to his time with the Atlanta Hawks and executives Rick Sund and Danny Ferry and coaches Mike Budenholzer and Kenny Atkinson.

This season, Horford, who is averaging 10.9 points and 6.3 rebounds, has held down the inside fort as the Celtics await the return of defensive anchor Robert Williams III from knee surgery that took place just before training camp.

Horford said it was important to him to clear up lingering uncertainty about his future, and to show a commitment to the Celtics, who enter Friday with a league-leading 18-4 record.

"I think this was something that was going to happen at some point or another," Horford said when asked if extension talks had been in the works. "Like I said, there were different things that the team had to deal with and things like that. There was really no urgency, per se, but I wanted to make sure that this didn't drag out until free agency and the summer and all those things and just once the time was right, to work this out and just focus on the season."