Why do people think that relationships are only about controlling the other person?

Its not controlling but if you be good with people and show real love towards them they will surely listen to what you say so there will be no need for you to controll them..

that's a very sad way to think of a relationship. I'm thinking it's more like discovery and opening of body, heart, and soul.

if you sad about being controlled it actually means you would be happy if you are in control

Because those people have warped thoughts, they're insecure, they don't know what it takes to have a good and healthy relationship, or all of the above.

Because they view relationships as "one-sided".

because most are

Who think that, that person is very wrong, seriously

Thats the way immature insecure people treat them.