Frei aber einsam - Brahms, his pact with Joachim - his gentle soul

FAE - "Frei aber einsam" was a pact that Brahms made with Joseph Joachim, his close friend, who was a violinist (also playing the viola). Of course it was a romantic idea of remaining free from the bonds of marriage and other social institutions that would jeopardize their artistry. But unfortunately, to Brahms' initial dismay, Joachim did meet the love of his life, an opera singer named Amalie Schneeweiss, and married her in 1863. Brahms for his part had taken their pact quite seriously. 

But, as it was, Joachim was tormented by jealousy and finally divorced Amalie, accusing her of adultery in 1884. This is when these two songs were written. Brahms apparently was one of few who defended Amalie. In writing these songs he might have wanted to remind Joachim of his loving wife's side to the story (having borne him 6 children). The 2nd piece, "Geistliches Wiegenlied" indeed starts with the viola and is reminiscent of the Christmas song "Joseph, Lieber Joseph Mein", playing its exact melody.

The 1st verse of the Christmas song:

Joseph, lieber Joseph mein,
hilf mir wiegen mein Kindelein,
Gott, der wird dein Lohner sein
im Himmelreich, der Jungfrau Sohn Maria.
Eia! Eia!

Johannes Brahms Op. 91, zwei Gesänge für Altstimme, Viola und Klavier

Perhaps Joachim was experimenting more with the viola at the time. In any case they were dedicated to him.

Here one of my favorite singers interpreting the songs 

with Wolfram Christ (viola) and Daniel Barenboim (piano).