Happiness serves hardly any other purpose than to make unhappiness possible.

The remark expresses a characteristically Proustian, bittersweet psychology: joy is not self-sufficient but functions as a contrast-agent that sharpens later pain. In this view, happiness creates expectations, attachments, and a sense of what life can be; once those conditions change—through loss, jealousy, time, or disillusion—unhappiness becomes not only possible but more acute because it is measured against remembered pleasure. The line also implies that emotional states are relational rather than absolute: we recognize suffering partly because we have known its opposite. It fits Proust’s broader preoccupation with memory and the way past delights can intensify present grief.