Wisdom from the Early Stoics, Zeno of Citium 54

An animal's first impulse, say the Stoics, is to self-preservation, because Nature from the outset endears it to itself, as Chrysippus affirms in the first book of his work On Ends. 

His words are: "The dearest thing to every animal is its own constitution and its consciousness thereof"; for it was not likely that Nature should estrange the living thing from itself or that she should leave the creature she has made without either estrangement from or affection for its own constitution. 

We are forced then to conclude that Nature in constituting the animal made it near and dear to itself; for so it comes to repel all that is injurious and give free access to all that is serviceable or akin to it. 

—Diogenes Laërtius, 7.85