How can people clinch their tongues in a constant fist?

Now here's something I'm actually asking.
I'm the type of person who says what's on my mind & if pl don't like wht I say, who cares(then again I sometimes epeack before I think xd)
How can people stand not saying what's on their mind?
Why? I mean, I could see on certain occasions, but why on a day to day basis, letting their voice go unheard?

just like you can speak your mind
others cant
its the same thing for them
you find it weasy to speak your mind
they find it easy to not speak theres and just let it go un-heard
some reasons for that are \
because there shy/nervouse
dont want to upset people
dont feel the need/cant be bothered
have a fear of public speaking even
o even other more seriouse problems
depression can cause it too a bit
theres lots of reasons, and it affects a lot of people

I think Jaz was pretty close to the marker. People just care o much about what everybody else thinks and they don't want anybody to be mad at them at all.

I don't really speak my mind just for it's usually more polite. I don't think about myself I think of everyone else not what they think of me but if it would hurt them. I don't like causing people to be less than happy.

Not easily so I believe