Or perhaps it's because baseball might only be his third best sport? In 2003, Antonelli was the Massachusetts Player of the Year in both football and hockey, but only the runner-up in baseball.
Is it due to a stellar college career at Wake Forest – where he batted .333 in 219 at bats – after which he was drafted 17th overall by the Padres and received a $1.5 million signing bonus?
Maybe it's because he just turned twenty-three (although he looks about fourteen) and he's the number two prospect in the Padres' organization. But then again, perhaps it's a nervous smile; surely being such a top prospect adds a lot of pressure?
"Not really," he says. "I don't really look at the prospect list or anything like that. I'm just going to keep trying to do whatever it takes to get up there [the majors] and not worry about if I'm number two or five or ten or whatever prospect it is, just try to get up there."
Is that smile the result of his terrific 2007? After the promotion to the Double-A San Antonio Missions, Matt Antonelli batted .294 with a .395 OBP and helped lead the team to the Texas League championship.
"[I'm] just trying to…hopefully improve on all the things I did last year and just get a little bit better at everything." (Perhaps it's a smile derived from modesty?)
Could it come from his versatility, and the confidence he has that you could put him almost anywhere and he'd succeed? Originally a third baseman, he's also spent time at short and some believe he could play all three outfield positions.
Might that smile be a result of his promising new team and its excellent blend of players?
"Everyone seems excited and ready to go play," he says. "It seems like a good mix of older guys, some veterans, and some younger guys."
And – although he's not known for his speed – maybe that smile is for those triples he hit in each of his first two games with the Beavers. (In his previous 191 games he had only seven).
Perhaps it's due to his manager Randy Ready who has nothing but good things to say about him.
"Matt's a refreshing young man. He's got all the right tools to play at the major league level with his makeup included. Strong work ethic, great attitude, his ability to apply information is what's propelling him. This guy loves to play."
Most likely the answer comes from a combination of all of the above. Wherever he ends up, whatever position he ends up playing, Matt Antonelli seems destined for big things. And that ever-present smile tells you he's enjoying every minute of the ride.