I think the best answers to the OP’s question are those given by Cessandra and by the terrific article that Murcielago linked to. (Other posters have made some good points too.)
I would add that it’s hard to let people know that you care about them or are interested in them or feel positively toward them without talking to them. But those of us who have gaps in our social skills or are out of practice don’t always know how to do this, at least without coming off as clumsy or intrusive.
I am a Quiet Person. If you asked people who know me to describe me, this is probably the first thing they would come up with. It is, I believe, my biggest flaw. I hear people who can just talk and talk and talk, especially if they can manage to stay interesting, and I think, “How do they do that! I wish I could do that!” If I could change one thing about myself, it would be to make myself a better conversationalist, more easily able to talk to people. If only…—then maybe I could make friends, get dates, make connections, enjoy people’s company more, and have a better life.
I think it was sometime in high school that I had the epiphany that, if I wanted to have friends, people would have to get to know me, and for that to happen, I’d have to talk to people. Duh! But knowing that I should talk to people, and knowing how to/being able to, are two different things, and though I’ve gradually gotten better over the years, I still have a long way to go.
As that article says, introverts “tend to think before talking, whereas extroverts tend to think by talking.” Yep. Before I can speak, I have to think of what to say, and then how to say it, and by the time I’ve got the words all lined up, the opportunity has often passed. (If only you knew how long I had to think this over before writing/posting it!) People who do their thinking out loud, out in public, are easier to get to know.
Oh, and sometimes the quiet person is the one whose interests or experiences don’t coincide with the rest of the group (like a non-fan in a group of sports fans, a non-parent in a group of parents talking about their kids, or an outsider in a group of professionals talking shop). Then the person who doesn’t say much because he doesn’t have anything much to contribute is seen as different (which he is, in a sense), uninterested and uninteresting, and maybe weird or stuck-up or elitist.