"I have to look after one kid at home," Thornton, who recently became a father, told NHL.com, "and now I have to look after 22 on the road."
Thornton was just 23 when he was captain of the Bruins from 2002 until his trade to the Sharks in November 2005.
"You grow as you age, and you kind of mature as a player," Thornton told the San Jose Mercury News Thursday morning. "I think you become more comfortable in your skin. Back then, you maybe let your game speak louder than your words and I think now you consider both. You can stand up and tell guys how it is or just go out and play."
"We had a really good assortment of choices," Sharks coach Todd McLellan told NHL.com. "It basically came down to two players. We just felt right now that Jumbo [Thornton] would be the right guy at this time. He's made some real big strides over the last couple of years. He changed the way he approached and the way he played the game. This is the next step in the evolution."
Pierre LeBrun covers the NHL for ESPN.com.