More than we might expect, at least from such a self-critical composer as Brahms, his catalog – as noted earlier – includes various reworkings of prior compositional efforts. (The Piano Trio in B major was revised more than 30 years after it appeared in 1854, but it retained its designation as Op. 8.) The last published of the three piano quartets (1875) was in fact his first attempt at the form, dating from the mid 1850s, when Brahms was in his early 20s and smitten with pianist Clara Schumann, but the music was set aside for two decades.
So far as research can tell us, the original finale was revised to become the scherzo (with no central trio section) and a new finale was written, as was an entirely new slow movement. The work had been drafted in the key of C-sharp minor, but Brahms opted for C minor in the revised version. (That key was much on his mind at that point, since his long-delayed First Symphony (premiered 1876) and the String Quartet No. 1 (1873) were also in C minor.
Suffice it to say that the work overflows with passionate outpourings of frustrated love and emotional turmoil, which led Brahms to suggest that the work be published with an illustration that would link it to Goethe’s Sorrows of Young Werther, in which the title character commits suicide because of his love for the wife of a friend.
— Dennis Bade