Bernard began preaching on the Song of Songs to his monks around 1135 and was still at it when he died in 1153. Eighty six sermons survive, and in the whole of that span he reached only the opening chapters. The first several are spent on the first half of the first verse.
The slowness is the method. He reads the Song as the love of the soul, and of the church, for the Word, and treats every phrase as an occasion to describe an interior state precisely: the difference between the kiss of the feet, the kiss of the hands and the kiss of the mouth as three stages of penitence, service and union; the way consolation arrives and withdraws without warning; the honest admission that he does not know how the Word comes to him or how it leaves, only that it has been there.
His short treatise On Loving God sets out the same movement in four degrees: love of self for self's sake, love of God for self's sake, love of God for God's sake, and love of self for God's sake, the last of which he says is barely reached in this life.
The combination is what made him so widely read. This is affective, first person, sometimes erotic writing about God, produced by a man who was simultaneously running an order of hundreds of houses, arbitrating a papal schism and dictating letters to kings. Later mystical writers in the Latin West work in the space he opened, and the Cistercian preference for describing experience rather than systematising it begins here.