Today is the feast of St. Catherine of Siena, Doctor of the Church. (She comes across as a very serious saint, but Euphrosyne, which means 'joy' or 'merriment', was apparently a family nickname for her, because she was always laughing as a child.) From one of her letters, to Brother Matteo di Francesco Tolomei, OP:
Dearest son in Christ sweet Jesus: I Catherine, servant and slave of the servants of Jesus Christ, write to you in His precious Blood, with desire to see you seek God in truth, not through the intervention of your own fleshliness or of any other creature, for we cannot please God through any intervening means. God gave us the Word, His Only-Begotten Son, without regard to His own profit. This is true, that we cannot be of any profit to Him; but the reverse is not the case, because, although we do not serve God for our profit, nevertheless we profit just the same. To Him belongs the flower of honour, and to us the fruit of profit. He has loved us without being loved, and we love because we are loved: He loves us of grace, and we Him of duty, because we are bound to love Him. We cannot be of any profit to God just as we cannot love Him of grace, without duty. For we are bound to Him, and not He to us, because before He was loved, He loved us, and therefore created us in His Image and Likeness. There it is, then: we cannot be of any profit to Him, nor love Him with this first love. Yet I say that God demands of us, that as He has loved us without any second thoughts, so He should be loved by us. In what way can we do this, then, since He demands it of us, and we cannot give it Him? I tell you: through a means which He has established, by which we can love Him freely, and without the least regard to any profit of ours; that is, we can be useful, not to Him, which is impossible, but to our neighbour. Now by this means we can obey what He demands of us for the glory and praise of His Name; to show the love that we have for Him, we ought to serve and love every rational creature, and extend our charity to good and bad, to every kind of people, as much to one who does us ill service and criticises us as to one who serves us. For God is no respecter of persons, but of holy desires, and His charity extends over just men and sinners.
I find this line of thought fascinating: That, because we have a duty to love God Himself, our way of loving God freely and as it were by grace is to love our neighbor. (Obviously we have in general a duty to love our neighbor, but I take it that the point is that in the case of our neighbor we can find ways to be useful to our neighbor, which means we can construct and select the ways we do it that are relevant to each and are profitable to them, for the love of God.)