Hitler had very little choice. England was being fed and armed by American convoys. Eventually, Hitler knew he would have another active front from a revitalized England. Hitler knew he couldn’t win in Russia and sustain an invasion in France.
Hitler could only slow that by sinking our shipping as it was coming out of American ports before it formed into convoys. Doing so would cause us to declare war, so it was a moot point.
Hitler only needed another year of determined invasion of Russia to secure that front. He was buying time by attacking American shipping.