Walker, who played eight seasons with the Charlotte Hornets, explained one reason he left the team that drafted him ninth overall in 2011.
"I gave that organization every single thing I could give them," Walker told Scott Van Pelt on SportsCenter (h/t ESPN's Tim Bontemps).
"Now it's about competing at the highest level, and we all know Boston is doing that year in and year out. I want to be part of something special in terms of competing at the highest level."
Walker excelled in Charlotte, averaging 19.8 points and 5.5 assists over eight seasons. He's coming off a career-high 25.6 points per game.
However, Charlotte has largely struggled this decade, making the playoffs twice during Walker's tenure and failing to win a postseason series. That isn't Walker's fault, of course, but the Hornets have encountered serious trouble taking the next step.
Boston doesn't look like a strong title contender next year, especially with center Al Horford now a member of the Philadelphia 76ers, per Wojnarowski.
However, the 49-33 C's still have a solid core of young talent led by top-three overall draft picks Jayson Tatum and Jaylen Brown as well as four 2019 draft picks in Romeo Langford, Grant Williams, Carsen Edwards and Tremont Waters.
Walker will undoubtedly be trusted with leading the team on and off the court as its top scorer and one of its most experienced veterans.
At any rate, Boston still looks like a clear playoff team, giving Walker a better chance to compete at a higher level. The Celtics are 18-1 to win the 2020 NBA Finals, per Caesars Palace.