Well KI suppose it's because he's a cool and silent type. That's his way of a ninja! :D

Hard pick o_o Maybe bacause he's thinking? Maybe because he's a ninja. I'd just go with that, that's the way he is. Like, he was born like that x) Coooool!!

Because I don;t know because he is cool.And not opening his cute mouth like the others who are saying something which has no pint!

I'm agree wit my sis.Kyara,in naruto is Sakura silent?No.But she is a ninja too.So i thing mabe because he thinking?It's just his character!Nothing more.He likes do be so and to show he IS a strong brawler?!..maybe...O.o

Faby I think Kyara meant because he's a silent type ninja. You see Naruto is a whole different ninja anime. All the characters in there are ninjas, it's a ninja life. But "In real life" in ninja stories ninjas are silent and calm. They walk in the night so no one can see them nor hear them. That's why they are ninjas.
And what could Shun thinks bout, might you tell me? He can't be always thinking about SOMETHING in all the episodes. He always stays calm, silent and with his eyes closed. So how can you think he i always thinking. Maybe it's his personality but really not thinking. How can he thing in every single episode then? That's strange...
The real answer IS that he is a ninja and ninjas are the same type.
Naruto is a whole different ninja anime. Their ninjas are different from the other ones.