It is in the nature of man to look forward to better times, despite set-back and disappointments.
The proverb is a quotation from Pope‘s Essay on Man (1733).
Hope springs eternal in the human breast:
Man never is, but always to be, blest.
The soul, unesay and confin’d from home,
Rests and expatiates in a life to come.
The second line means that man never is blissfully happy, yet always thinks he is about to be. In the fourth line ‘expatiates’ means ‘wanders freely’.