Joy, sadness, desire: the three primary affects
Formal Statement
The mind can pass to a state of greater perfection (joy/pleasure) or lesser perfection (sadness/pain). Together with desire (conscious appetite, III.P9 Note), these constitute the three primary emotions. Spinoza recognizes no other primitive affects: "beyond these three I recognize no other primary emotion." Every other emotion is a compound or variation of these three.
In Plain Language
All of human emotional life — love, hate, jealousy, pride, shame, hope, fear — reduces to three building blocks. Joy is the transition to greater power. Sadness is the transition to lesser power. Desire is the conscious striving itself. Love? That is just joy accompanied by the idea of an external cause. Hate? Sadness plus an external cause. The entire baroque complexity of human feeling is, for Spinoza, a combinatorial explosion from three primitives. This is the emotional periodic table.
Why This Follows
From ce-13, the mind registers increases and decreases in bodily power. Spinoza names the transition to greater perfection "joy" and the transition to lesser perfection "sadness." Combined with desire (ce-11, the conscious conatus), these three exhaust the primitive affects because they correspond to the only possible directions of power-change: up, down, and the baseline striving itself.
All emotions are compositions of three primitives: joy (power up), sadness (power down), and desire (conscious striving).
Pick a complex emotion — say, nostalgia or jealousy. Can you decompose it into some combination of joy, sadness, and desire?