He is a fool that is not melancholy once a day
Gurus abound with good advice on staying positive, banishing negative thoughts and promising everlasting happiness.
So why did the sage who coined this saying think being sad was a good thing?
It raises the question what is sadness, and what is it for?
A spell of melancholy is a time for reflection on the bad things in your life. You could simply try to ignore them, suppressing all thoughts. You could pretend they don't exist. At worst you could be taking pills in an attempt to blot them out.
This sage is encouraging us to face up to our difficulties and attempt to deal with them. After the thunder and rain we get the sunshine and it is them we can enjoy the benefits of a positive attitude.