Yes, will say "no" backed up with good reasons, but not necessarily explaining oneself to the person who asks you for something. It really depends - on a variety of situations.

Basically, it's a confidence issue and the ability to rely on oneself to get things done in the event the person reacts negatively and chooses to withdraw their acceptance or support. If one can stand alone with a best friend or family by one's side, then it doesn't matter what other people think of you.

Anytime anyone criticizes you, always look further to see if it's motivated by bias or envy. Most of the time, it is. Very few people have self awareness also. Some criticisms are constructive but many are destructive.