Michael Eric Repper’s history of unflagging devotion to a narrow set of passions dates back to the early 1990s when, as a 3-year-old, he snapped to attention the moment the orchestra kicked in at a classical music concert. By the time he had reached his early 20s, another of his select few passions was consuming him: his relationship with his girlfriend, Vanessa Rodrigues Moody.

Dr. Repper, now 33, became the youngest American to win a Grammy Award for Best Orchestral Performance this year, and Ms. Moody, 31, a lawyer with the global law firm Paul, Weiss, Rifkind, Wharton & Garrison, met and began dating in April 2013 as students at Stanford.

Six months later, when she was a senior and he had graduated and moved to Baltimore to start a doctoral degree in music, neither was sure what would become of their budding romance. But on April 14, 2014, she called to tell him she had been diagnosed with a rare brain tumor the size of a tangerine and asked whether he wanted out of the relationship. Both knew then it was built to last.

“I was terrified,” Dr. Repper said. “But I was also all in.”

Dr. Repper grew up in Laguna Niguel, Calif., with a younger sister, Danielle. His mother, Dr. Claudia Repper, is an emergency room doctor. His father, David Repper, designs employee training programs for corporations. But his maternal grandmother, Marilyn Smith, was musical: She played piano and, before he had reached kindergarten, brought him to children’s concerts at the Pacific Symphony in Costa Mesa, Calif.