Proposition XXXVII. The good which every man desires for himself he will also desire for other men, and so much the more, in proportion as he has a greater knowledge of (Deus sive Natura).
Men, in so far as they live in obedience to reason, are most useful to their fellow men; therefore we shall in obedience to reason necessarily endeavour to bring about that other men should live in obedience to reason. But the good which every rational man desires for himself, is to understand; wherefore the good which each rational man seeks for himself, he will desire also for others.
The more a rational man longs for the good the more he realizes that it is in his own best interest for all to do the same. For example, if one would live in peace that goal would most readily be achieved if all men also desired to live in peace. It is the way of reason is to want for others only what one wants for oneself.