The title of this post, Love Letters from a Rosh Yeshivah, is a fake-out. Sorry. I'm not sure who started the rumor that R. Nosson Kamenetsky's Making of a Godol contains a report about R. Aharon Kotler's love letters to his fiance, but that is entirely false. What follows is the entire short passage in the book and it is a far stretch to suggest that the letters are love letters. The suggestion of the author is that the letter discussed R. Kotler's popular lectures to fellow students in the yeshivah he was attending, an action that does not fit the highest rung of modesty. From the "Improved Edition" of Making of a Godol, p. 802 n. v:
A reliable source reported that R' Aaron wrote a letter to his fiancee of which her father, R' Isser-Zalman Meltzer, disapproved. When it was shown to the Alter, he rejoined, "I did not tell you he was a tzaddiq; I said he had other qualities. But his frumkeit will yet become so great that everyone will suffer his brunt." This author conjectures that the letter concerned an impressive haburah he had delivered -- see Ch. 3, beginning of n. 127... On June 26, 2003, R' Ya'aqov Hirschmann summarized R' Aaron's eventual frumkeit by repeating what Rebbetzin Kotler remarked to talmidim returning from her husband's funeral: "Everyone spoke about a great gaon who died; and I say a great tzaddiq passed away."