Johannes Brahms grew up in a family where his father (who was also a musician) and his mother were stuck in a loveless and sexless marriage. Just to confuse things further for a young man, Brahms was got a job as a 12 year old working in a brothel entertaining the patrons and so he got an eyeful of the seedier side of sexual relations. Hence at home it was the staid marriage of his parents contrasted against the other extreme he saw at work.
So when a 25 year old Johannes met and fell in love with Bertha Faber who sang in the choir he directed, he didn’t know how to reconcile his experiences with intimacy. They took long walks together and spent many hours talking but then Brahms left her suddenly but not before he had commissioned to memory a little tune she would always sing to him during their time together.
10 years later he saw Bertha again on a Vienna street except that this time she was hanging on the arm of a wealthy business man. By this time Brahms was a respected composer and upon seeing Bertha (who was pregnant with her second son) he remembered her little melody and used it as inspiration for his now famous lullaby which he dedicated to her newborn baby. When you sing Brahms Lullaby it fits perfectly as a counterpoint to Bertha's melody.
That baby boy was named Johannes.