"Just one of those things that it was a choice I made," Thornton said. "I felt with the workload I've had over the last four or five years and the way I felt at the end of the year, the way I was struggling in back-to-back outings, it was something in my best interest and the team's best interest to avoid this year.
"I loved the WBC. I had a blast. Getting to meet guys you normally don't get to play with and be a part of Team USA and that kind of thing, it was an honor. It was a tough choice. It wasn't easy for me to say no. It took me a few days to make that choice. I felt it was in my best interest to avoid."
At 36, Thornton begins his seventh year with the White Sox in quite possibly the best shape of his career. He enters 2013 as the franchise's all-time holds leader, fourth on the franchise's list of appearances at 472 and is the all-time appearance leader among White Sox relievers.
The White Sox have a $6 million option or a $1 million buyout for Thornton in 2014, but the southpaw wasn't worried about anything past the bullpen he threw Friday. As for how long he wants to play beyond this season, Thornton has an idea when it will be time to go.
"It definitely gets harder as your kids get older and stuff," Thornton said. "You see guys that I know now that have kids who are 8, 9, 10 years old. You hear Jake [Peavy] talking about how much he misses his kids all the time. It's a lifestyle that's not easy.
"At the same time, I love the game of baseball. I love competition and going out there and the preparation in the offseason. I still love working hard getting ready for the season. When I start to avoid the preparation part is when I think that's when I'll realize I'm ready to phase out. I had one of the best offseasons of my career this offseason.
"Obviously, wearing the White Sox uniform for rest of my career would be something I would love to do," Thornton said. "But that's definitely out of my hands."