Niceness is a measurement of how kind or pleasant others perceive you to be.
Happiness is a measurement of your internal state.
Nice people can be unhappy and happy people can appear to be unkind.
People who say “yes” to everyone, because they don’t want to hurt anyone’s feelings, are a common example of how being happy and being nice can come apart; such people are frequently tormented with feelings of guilt, resentment, and frustration. Yet, they are usually regarded by everyone as nice.
Conversely, some of the happiest people in the world are thought of as unkind because they set strong boundaries and demand respect from others. Since it’s impossible to say “no” without rubbing SOMEBODY the wrong way, all happy people must make choices that will cause someone to see them as unkind (even when happy people go out of their way to prevent this).
This is not to say that happiness and kindness are completely unrelated.
Some people are kind because they’re happy and some are happy because they’re kind.
Also, some people are unkind because they’re unhappy and some are unhappy because they’re unkind.
Nevertheless, the two are not identical.
So, remember to keep the distinction clear.
You’re nice when others are at peace with you. You’re happy when you’re at peace with yourself.