Saint Bernard of Clairvaux

"Therefore my very dear brothers, preserve peace among you, and beware of offending each other, whether by deed or word or any gesture whatever, lest someone, provoked and surprised by passion in a moment of weakness, should be constrained to invoke God against those who injured or saddened him, and impetuously cry out this grave accusation: "My mother's sons turned their anger on me." For those who sin against a brother sin against Christ who said: "In so far as you did this to one of the least of these brothers of mine, you did it to me." 
Nor is it enough to avoid only the more serious offences, for example, public insult and abuse or the venomous slander in secret. It is not enough, I say, to guard one's tongue from these and similar kinds of nastiness; even slight offences must be avoided, if anything may be termed slight that is directed against a brother for the purpose of hurting him, since merely to be angry with one's brother makes one liable to the judgment of God. And justly so. Because what you regard as slight, and therefore commit with all the more ease, will be seen in a different light by another, just as a man looking at the outward appearance and judging according to the outward appearance, is prepared to think a splinter to be a plank, and a spark a blazing fire.
The love which believes all things is not the gift of all men. A man's heart and thoughts are more prone to suspect evil than to believe good, especially when the obligation of silence does not permit you, whose conduct is in question, to defend yourself, nor him who suspects you to lay bare the wound from which he suffers, that it might be healed. And so he endures the agony, grieving in his heart, till he succumbs from the secret and deadly wound, totally immersed in anger and bitterness, his mind a whirl of unvoiced thoughts on the injury he has received. He cannot pray, he cannot read, nor meditate on anything holy or spiritual. And while this soul for whom Christ died is cut off from the vital influence of the Spirit, and goes to its death through lack of the nourishment it needs, what, I ask, are the thoughts of your own mind in the meantime? What can you find in prayer, or in any work you do, when Christ is sorrowfully crying out against you from the heart of your brother whom you have embittered, saying: "My mother's son is fighting against me, he who enjoyed my meals with me has filled me with bitterness." 

St Bernard of Clairvaux: from Sermon 29 on the Song of Songs

Jesu dulcis memoria, words attributed to St Bernard, a setting (also controversially) attributed to Tomas Luis de Victoria, sung here by the Cambridge Singers 




And an interesting recording of Thomas Merton OCSO speaking on the subject of love in the theology of Saint Bernard of Clairvaux: