Instead of the usual sadness and gloominess, she felt enthusiasm and cheerfulness!

Cheerfulness in a business letter equates to using a positive attitude.

Cheerfulness should not mean frivolousness, cheerfulness should be used in a professional manner.

Even a letter that has a negative purpose or delivers bad news can be worded from a positive point of view.

After the man received the trophy, he was in a cheerful mood for the rest of the day.

Example sentence - We were grateful for his cheerfulness after the funeral.

cheerful, cheerless