Thank you! 

It's Melkor defending his lieutenant and dear husband, as well as also his own pride as a married entity in Arda who feels that like his marriage, his other accomplishments and qualities are often mischaracterised or swept under the table because he happens to not vibe with the "Eru crowd". And by writing to The Professor™ he thinks he has found the most "civilised" method of airing his grievances to show that writer who he really is, lest he ponder the use of Grond again. (Picturing Melkor stomping through Oxford University with his Hammer of the Underworld and Opinion Amplifier in hand, frantically searching for Tolkien, is highly entertaining.)