I don't keep quiet, I am quiet.
I don't think people choose to be quiet or loud or talkative. I also don't believe you can train yourself to be different from your natural personality type.
I don't choose not to speak, I really have nothing to say and I don't notice things. If there is something I am very interested in, or very knowledgeable about, I can talk about it or ask questions about it, but most of the time, I don't have any interest in the things most people talk about, and I don't notice most social cues, so I have nothing to talk about.
It's like how some people can go on vacation to a place and talk nonstop for days about their vacation after they return, and everybody wants hear about what they saw, what they did, etc. I don't. If I want to go there, I'll go there myself. If I want to learn about a place, I'll pick up a book or research it on the net. For me, the only experience, is to actually experience something or some place. I just don't get anything out of talking about things, and see no point to it.