Proposition. XXXV. If anyone conceives, that an object of his love joins itself to another with closer bonds of friendship than he himself has attained to, he will be affected with hatred towards the loved object and with envy towards his rival.
Note.—This hatred towards an object of love joined with envy is called Jealousy, which accordingly is nothing else but a wavering of the disposition arising from combined love and hatred, accompanied by the idea of some rival who is envied... This condition (of hatred toward a rival) generally comes into play in the case of love for a woman: for he who thinks, that a woman whom he loves prostitutes herself to another, will feel pain, not only because his own desire is restrained, but also because, being compelled to associate the image of her he loves with the parts of shame and the excreta of another, he therefore shrinks from her.
Proposition. XXXVI. He who remembers a thing, in which he has once taken delight, desires to possess it under the same circumstances as when he first took delight therein. This pain, in so far as it has reference to the absence of the object of love, is called Regret.
Pain diminishes or constrains a man's power of activity, in other words, diminishes or constrains the effort, wherewith he endeavours to persist in his own being... Again, since pleasure increases or aids a man's power of activity, (therefore) a man affected by pleasure has no desire further than to preserve it, and his desire will be in proportion to the magnitude of the pleasure.
Lastly, since hatred and love are themselves emotions of pain and pleasure, it follows that the endeavour, appetite, or desire, which arises through hatred or love, will be greater in proportion to the hatred or love.
Proposition. XXXVIII. If a man has begun to hate an object of his love, so that love is thoroughly destroyed, he will regard it with more hatred than if he had never loved it, and his hatred will be in proportion to the strength of his former love.