Why is being quiet not a good thing?

Because if some people are quiet they could be holding in something serious or if you are in class and you are always quiet your peers and teachers are going to wonder if something is bothering you 
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Is that from a story? How quiet?

Well, this is as much as I could answer. Being VERY VERY quiet isn't a good thing because...
1. You can't stand up for yourself. Even if you have all the mind-power but you can't say it out, you will look weak and wimp in the eyes of bullies or other people. 
2. If you know/saw something very bad that happened, and you have the urge to tell, but keep your mouth shut cuz u are just that type of "quiet" person. That is also bad because you can't speak out and provide evidence... for example.. a man saw a robber rob a guy. A cop notices that and starts running there to stop it. The robber quickly puts the mask on the guys (that he was robbing/victim) and gives him the gun. The cop, when he comes will obviously take on the victim guy, cuz he looks like the actual robber. The real robber will appear a lamb, and get his victim in trouble instead of himself. The man standing in the distance, saw all of this, and didn't tell a thing/testify against the "good" robber. This also relates if you are the guy that got robbed, and you are super quiet and shy/bla bla bla. And you can't testify for yourself and prove that you are clean... just because you are quiet.

These are just two examples why it isn't good to be TOO quiet. But sometimes you have to be quiet, ad not stand out... so you don't get into any trouble...