What is the meaning of [It is a sad heart that never rejoices]

Sometimes ‘poor’ is used instead of ‘sad’. Even the most confirmed of
pessimists can’t be miserable all the time. We can’t look for ever on the
gloomy side of things.

> Mrs Stevens looked up from the letter she had been reading at the breakfast
table.
> ‘It’s from Mother,’ she said to her husband. ‘She can’t come and stay with
us after all.’
> ‘It’s a sad heart, ‘ said Mr Stevens from behind his paper, ‘that never
rejoices.’