Every man exists by sovereign natural right, and, consequently, by sovereign natural right performs those actions which follow from the necessity of his own nature; therefore by sovereign natural right every man judges what is good and what is bad, takes care of his own advantage according to his own disposition and endeavours to preserve that which he loves and to destroy that which he hates.
Now, if men lived under the guidance of reason, everyone would remain in possession of this his right, without any injury being done to his neighbour.. But seeing that they are a prey to their emotions they are often drawn in different directions, and being at variance one with another stand in need of mutual help.
Wherefore, in order that men may live together in harmony, and may aid one another, it is necessary that they should forego their natural right, and, for the sake of security, refrain from all actions which can injure their fellow-men. The way in which this end can be obtained should be able to render each other mutually secure, and feel mutual trust, is ... through fear of incurring a greater injury themselves.
Spinoza agrees that each man is entitled to act in his own best interests. But men who are guided by reason know that they need the help of their fellow men therefore they will curb their actions so that they will not hurt their fellow men. The emotion which can overcome the desire to act as an individual is the emotion of fear generated from the knowledge that hurting others is really, in the long run, hurting oneself.