Quiet boys are less vocal about things. It's what makes them quiet. But this can turn out to be a negative thing when it comes to issues that they have with someone they're involved with. Problems and issues that they have with people could get tucked away with the hope that things will pass. But without proper conversation about these things, it's likely that they'll just linger and after enough time the problems that were just small issues will turn into major difficulties.
Because we few with the quiet confidence are the ones that see everything. We see right into your soul. We see all your strengths and weaknesses, we pick you apart. If you try to toy with us or manipulate us we know just how to get you, how to manipulate you, or how to hurt you. We are like fire, if you treat us with respect we will keep you warm, but if you play with us you are going to get burnt.
I think the idea is that you can't ever tell what they're thinking. As Maddox said, think about people who go on shooting rampages. It's always "that quiet guy who always kept to himself", and never "that loud motherf***er always making a racket".
But as always, quiet and social don't necessarily mean anything.