I'm pretty introverted, though not really shy. I have no issues going out, and hate days (like today) where I don't see other people at all.
But, I also like to do my own thing. This kinda comes at the bafflement of my friends, who see me as friendly and outgoing one minute and then don't understand why I suddenly disappear or outright ignore them the next couple times they try and get in touch with me.
For me, being around other people for a couple hours a day is great. So, yeah, I love getting meals with people. I love hanging out, going out drinking, etc. I'm even cool with the occasional (weekly, perhaps) day out with others. All of that's great. But when that same person starts coming around all the time, I start to feel intruded on and the impulses to shut-off start kicking in. Then people get sad, because they think I'm mad at them or something. I'm not, I just need time to myself to do my own things. When I am out, I'm seldom the one doing the talking. I'm usually listening, or asking question. Rarely do I go off talking about myself. I dislike that and try to avoid it, as it's a quality that I really dislike in others as well.
Funnily, some of my friends here in HK have decided that, because of my semi-introverted behavior, I'm in-fact not an American but an Asian. Some of the others, who are not comfortable with the idea that a white dude could be for the most part like them on the inside, have compromised by saying that I'm not an Asian, but an Austriallian