The greatest commandment is not the first in a list but the center of them all, the center of everything, the law guiding all things. Love of God (which incorporates and appropriates His Love for the others) is the Law above all laws! We could say that it is on another dimension which includes "thou shalt" and "thou shalt not" but is more properly on the level of attraction, desire and natural inclination (the noble source at the bottom of every good human aspiration and ideal). In that the Romantics were right! They were wrong in forgetting the ten commandments which are also necessary guides because our nature has been corrupted by sin.
The human person is hard wired for love, and not just any love, but true, unconditional and selfless and immense (limitless), which is the love of God. That is what is meant by the first commandment first of all, that it is the law underlying all of existence and all reality, especially the reality of man. Man is oriented toward love and only love, perfect love, gives him fulfillment and peace.
Cf. Pope Francis Angelus message yesterday; and Saint Paul...
"My love is my weight!" --Augustine (Of which Archbishop Chaput reminded us in his outstanding and bold Address last Monday).
"Love God and do as you please." --Augustine
The use of the word "spirit" below seems to mean both the Holy Spirit and the spirit that inspires every good action in men!
Living by the Spirit
16I say then, walk in the spirit, and you shall not fulfil the lusts of the flesh. 17For the flesh lusteth against the spirit: and the spirit against the flesh; for these are contrary one to another: so that you do not the things that you would. 18But if you are led by the spirit, you are not under the law. 19Now the works of the flesh are manifest, which are fornication, uncleanness, immodesty, luxury, 20Idolatry, witchcrafts, enmities, contentions, emulations, wraths, quarrels, dissensions, sects, 21Envies, murders, drunkenness, revellings, and such like. Of the which I foretell you, as I have foretold to you, that they who do such things shall not obtain the kingdom of God. 22But the fruit of the Spirit is, charity, joy, peace, patience, benignity, goodness, longanimity, 23Mildness, faith, modesty, continency, chastity. Against such there is no law. 24And they that are Christ's, have crucified their flesh, with the vices and concupiscences.