Why Did Kristen Kreuk Say That Tom Welling Is “closed Off” In An Interview? And What Does That Mean?

I like kristen kreuk..I think she is quirky and weird and girly + a good role model. But I read an interview: http://www.rollingstone.com/news/story/5939764/tales_from_smallville
It specifically quotes Kristen talking about her on-set friendship with Tom Welling and she says:
“Tom and I get along very well,” says Kristin Kreuk, “but he is very closed off with people.”
1) What exactly does “closed off” mean? And why do you think she would say this? (I thought it was kind of weird to say that, b/c maybe she just needs to open up more to tom…)
2)If you read the whole interview…It is very interesting…Esp. Michael Rosenbaum’s responses…Kristen says she was view as sort of snobbish. Can anyone relate? I kind of feel like I can relate b/c I’m shy and I think being shy can come off as snobbish b/c you don’t say anything so people just think you dont want to, but its really b/c you don’t know what to say(shyness)…
—here is the part:
‘ Kreuk isn’t so sure about that. She thinks people have gotten the wrong impression of her. “You know why? It’s because I don’t open up very easily to people. In high school, I came across as a snob. Well, I am sort of a snob. I’m not very nice to people, because . . . well, I’m just not. I have a cold aura. So sometimes I can come across as mysterious.” ‘
Haha, I know I have big questions about my concerns, b/c I really like the characters in Smallville and the actors behind them, they all seem very different, and deep in some ways but also very outward…:)