For an excellent take on Program Notes, you must read my friend's blog post, "Programnotaphobia." I do hope that her blog isn't private, because she is hilarious, and spot on.
In spite of the very valid argument that music does not need program notes, particularly Brahms' music, since he was an advocate of "absolute" music (music that exists in its own right, with no stories or imagery attached), I'm going to share some of my own program notes about Brahms' Sonata No. 1 in G, op. 78, which is sometimes nicknamed, "Regenlied."
Like Beethoven, Brahms had a reputation for being brusque and sarcastic, and both men never married or had children. However, also like Beethoven, those who knew Brahms well found him to be warm, loyal, and very fond of children. He was known to take long walks out in nature, and he would often hand out candy to children if they crossed his path.
Two of the people who seem to have been the closest friends of Johannes Brahms throughout his life were Clara Schumann and Joseph Joachim. Clara Schumann was a concert pianist, and wife of the famous composer Robert Schumann, and mother of seven living children. Brahms had been introduced to the Schumann's through Joachim, a great violin virtuoso. It was for Joachim that Brahms composed his works for violin, including the G major Sonata, but he was also inspired by his devotion to Clara.
There is little doubt that Brahms and Clara Schumann cared for each other deeply, but it seems to have been a platonic relationship. When Robert Schumann tried to commit suicide, and was admitted into a mental hospital, he was allowed no contact with his friends or family, not even his wife. Brahms became the virtual head of the Schumann household, taking over all of Robert's obligations and responsibilities. When Robert eventually died, Brahms moved into an upstairs apartment so he could continue to help the fatherless Schumann family.
The first movement of the G Major Sonata begins with a three-D motif that appears throughout the work. In the final movement, that same three-note motif begins a quotation of a pair songs that Brahms wrote, Regenlied (Rain Song) and Nachklang (Tears). The songs are based on two poems by Klaus Groth which depict rain and the emotions that the rain invokes: remembrance of past joys in the first poem, and sorrow as the rain mingles with the poet's tears in the second.
That Brahms included an emotional message in this Sonata is confirmed in a letter he wrote to Clara around the time of her son Felix's death. (Felix was a violinist, and Brahms' godson as well.) He inscribed the main theme of the second movement, the Adagio, and wrote, "Dear Clara, When you play what is written...you will understand far more deeply than I can express in words how tenderly I think of you and Felix and his violin-even though it is silent."
In response, Clara is quoted as saying, "[I] could not help bursting into tears of joy over it. ... I wish the last movement could accompany me to the next world."