Why should expressing what you are thinking and having your own opinions be against the law?

It shouldn't AS LONG as your expression of your opinions do not harm others. For example, shouting fire in a crowded building or directly calling for violence against other where there is a good chance someone would commit violence because of your call.

For example, saying Trump should die is not really a call to action, but saying to an angry crowd to go attack a guy walking by, would be.

In countries where free expression is suppressed, it's mostly about those in power maintaining that power.

For the most part, it isn't.

However, the courts have decided on first amendment exceptions.

Sexual Harassment

Sexual harassment is a form of sex discrimination that violates Title VII of the Civil Rights Act of 1964.

https://www.aauw.org/what-we-do/legal-resources/kn...

The courts decided a boundary can get crossed when one's selfishness interferes. Victims have rights too.

It isn't

If your opinions contradict Leftist doctrine ( especially when you make good sense ) you must be silenced .

It shouldn't. In fact the right is guaranteed in the US Constitution. Many countries are not so fortunate.