Whoever knows my commandments and keeps them is the one who loves me. The foremost commandment is: Hear, O Israel: the Lord your God is one God, and you will love the Lord your God with all your heart and with all your soul and with all your mind. This is the first and the greatest of the commandments. But you cannot observe it unless you love your neighbor, for whoever does not love his brother whom he has seen cannot love God whom he has not seen; and so the second commandment is like the first: You will love your neighbor as yourself—that is, in the same way as you must love yourself and for the same reason.
Now what you must desire for yourself are those things which are truly good, not evil. If you wish yourself evil you are hating, not loving yourself, for whoever loves wickedness hates his own soul. This is the way, then, in which you must love your neighbor as yourself, wishing him good, not evil, for whatever you want others to do to you, you must do the same to them, and you should never do to another anything you would hate to have done to yourself by another. Love never wrongs a neighbor.
What you are to love in your neighbor, then, and do to him, are the things that will make him good if he is bad, or encourage him to persevere in virtue if he is good. Now it is for God’s sake, of course, not your own, that you must love yourself, for you turn the thing you love for its own sake into the ultimate object of your happiness and the crowning blessing of your whole life. All your present joy will consist in looking forward to its enjoyment. How unworthy it would be, then, for you to place your hopes for a life of blessedness in yourself, or in any other human creature! Woe to the one who puts his trust in man and relies on an arm of flesh, the one whose heart turns away from the Lord! It is the Lord you should take as the ultimate object of your happiness, it is to him you must look for a life of blessedness, for the apostle says: Now that you have been set free from sin and have become slaves of God, the return you get is sanctification, and its end, eternal life in Christ Jesus our Lord.
See things as they truly are, then, and you will find yourself obliged to love God for his own sake, and yourself not for your own sake but for God’s. And since you must love your neighbor as yourself, you will not love him either for his own sake or for yours, but for God’s, or rather you will love God in your neighbor, By this we know that we love God’s children, says the apostle John, when we love God and obey his commandments. If you love God for his own sake, and your neighbor as yourself for God’s sake, then you are doing all that is necessary to prepare your soul, for on these two commandments depend all the law and the prophets."
-- From the Book of the Institution of the First Monks