Why do people have such a hard time being open with their thoughts and feelings?

Because they have been reproached too many times for saying what is really on their mind, they have been bullied for being who they are, they have had people leave their side for what they believe or think.
It's a harsh world out there, people are mean. And it feels like speaking your mind sometimes comes with more backlash than its worth.

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Or sometimes it just comes across as TMI
Or we're too proud.

If you teach a child /person :

Criticism. They learn to condemn.

Hostility. They learn to fight.

Ridicule. They learn to be shy.

Shame. They learn guilt.

Tolerance. They learn patience.

Encouragement. They learn confidence.

Fairness. They learn justice.

Security. They learn faith.

Approval. They learn acceptance.

And most important. If you teach them kindness and acceptance. They learn to find love in the world. A very rare thing indeed.

Well with our modern technology, we are watched everywhere we go. You type or say something human, the mean people come out and scold, taunt and criticize you. Thats why one cannot be their true selves online or in public anymore. Thats my two cents.

Because they've been stuffed one too many times over it.
Or, it's something they've been conditioned to do as children.

The fear of being judged or misunderstood. Some just simply don't want to.

Because they can be painful, uncomfortable, embarrassing, confusing.

It has a lot to do with trust and self confidence.

I also believe that as a society we have become both comfortable with criticizing others while being simultaneously over sensitive to criticism.

Therefore, speaking one’s mind opens you up to unwarranted and ignorant criticism and everyone gets their panties in a bunch when criticized.

We need balance.

Probably because a lot of people here judge. Fingerpoint or see it as a game

a lot of (i'd argue most) people have been socialized to equate vulnerability or emotionality with weakness

I don't... hence all the "snowflakes" I offend and dumb people that block me... lol 😂

Very dangerous on Social Media especially when they request it.

Because most people don't care to hear them.

I do because someone's always telling me how I feel and what I want. Reality is I'm not sure what the hell I want at present time and I'm pretty frustrated!!!

I don't have the problem of opening up. My problem is that there is no one around to hear it. or care.

Because it means being vulnerable, risking feeling foolish or getting hurt, and most people are not that brave.