Why do we have to be “happy” or “sad”?

Why is there an “unhappy” that means sad but not an “unsad” that means happy?

Why can’t we ever be “unsad?”

Most of the time, that’s where I am.

Not happy. Not sad.

Not unhappy, though. Just alive.

Not doing great.

Nor bad.

But quite happy about it.

Or at least not upset.

Doing alright: