A jealous person is made unhappy by their own misfortune and by the happiness of others.
The quote suggests that jealousy breeds unhappiness in individuals. A jealous person often feels discontent not only because of their own shortcomings or misfortunes but also because they cannot find joy in the successes and happiness of those around them. Instead of celebrating others' achievements, they compare themselves unfavorably and dwell on what they lack. This creates a cycle of dissatisfaction, where their negative feelings towards themselves are compounded by resentment towards others, ultimately leading to a deeper sense of unhappiness.