Selection from – Ethics – Part III. On the Nature and Origin of the Emotions (Page 12)

Spinoza's Words: (preventing others getting what can't be shared)

Proposition. XXXII. If we conceive that anyone takes delight in something, which only one person can possess, we shall endeavour to bring it about that the man in question shall not gain possession thereof.

Proof.—From the mere fact of our conceiving that another person takes delight in a thing we shall ourselves love that thing and desire to take delight therein. But we assumed that the pleasure in question would be prevented by another's delight in its object; we shall, therefore, endeavour to prevent his possession thereof. Q.E.D.

Note.—We thus see that man's nature is generally so constituted, that he takes pity on those who fare ill, and envies those who fare well with an amount of hatred proportioned to his own love for the goods in their possession. Further, we see that from the same property of human nature, whence it follows that men are merciful, it follows also that they are envious and ambitious. Lastly, if we make appeal to Experience, we shall find that she entirely confirms what we have said.