The Battle of the Atlantic was ominous and melancholy; the music fits.
The "Happy Times" were not at all "happy" for the Allies and regardless of those brief periods of success, the long-term outlook for the U-Boats was doomed from the start. Doenitz knew that from the beginning...his comment on hearing that Great Britain had declared war says it all; "Dammit! That this has to happen to me again!" (but in German, of course).
He knew that he needed 300 boats in order to successfully prosecute his "tonnage war" and he started the war with 56; only 22 of those were "Atlantic Boats", the other 24 were the Type II "Canoes". It's astonishing what that meager force accomplished in the first year of the war. But, by mid-1941 the tide was turning and the technologies available to the Allies had already made all of the U-Boats in service and under construction, obsolete. Technological improvements to the U-Boat force were too little and/or too late from then on.
What is most ominous and melancholy is that, from mid-1943 on, they kept on going out in the face of hopeless odds against returning, let alone of scoring any real successes. The notion at BdU that they were "tying up" significant Allied resources was almost absurd...but it spurred them on...straight to a 75% casualty (fatality) rate. "Ominous and melancholy" is exactly what the music should be.