Being accepted is very important to us. It goes down to our roots, of if we don't fit in, we don't survive. This is still a familiar concept today. If we don't fit in, we can become depressed, reclusive, and have a low self worth. If we feel like our opinion will not be accepted, our instinct is to keep that opinion or thought secret. We will avoid talking about it so we have a better chance of being accepted socially. For example, if you see someone being treated wrongly or unfairly, but the people who are being the bullies are part of the group you want to be accepted in, it is very easy to ignore, or even worse, start doing the same thing.
Fear of embarrassment
It is pretty well agreed that embarrassment and shame are not great feelings. It can make you want to roll up into a ball and hide from everyone. When you speak up, you are risking being shamed. Very few of us purposely put ourselves in that type of situation, and will go to great lengths to avoid being embarrassed. So, it is logical that the risk of embarrassment is a pretty big factor in scaring us into pushing the problem under the rug. This clearly isn't a good thing, but once you step back and ask yourself, "Is what I want to say valid?" then slowly your confidence will grow and you will feel like you can take on the challenge.
It is easy to be quiet
When you see something happening that is wrong, for example, bullying, it is easy to think, "Yeah, that is wrong, but it doesn't really concern me." Or when you yourself are being bullied or insulted, you could think "Well, its not that bad, so I will just deal with it for now." It is very common for these thoughts to go through your head. We sometimes think that if we just ignore or tough it out it will end, but in most occasions, it doesn't get better, but worse. We also tend to think that no one will actually listen to us. But, even if it takes a little looking, there is someone who will genuinely listen to your opinion.