Rabbi Rosenshtain's mother was the shadchan (matchmaker) between their next door neighbor and R' Yerucham Levovitz, who would later become the mashgiach in the Mir Yeshiva.

At age nineteen, Moshe came home from yeshiva for Pesach and met R' Yerucham, with whom he became very close to in the ensuing weeks and months.

R' Yerucham was a major baal mussar (literally "master of ethics," here referring to someone who studies ethical works intensely and works tirelessly in self-improvement), a product of the Kelm Talmud Torah, and shared Torah thoughts with Moshe.

[1] In 1912, Rabbi Rosenstain was appointed mashgiach ruchani of the Lomza Yeshiva in Poland, a position he held for the rest of his life.

[2] In fact, he fasted for thirty years, eating only at night after completing maariv, and on Shabbos and holidays.