What is American music? And, perhaps more to the point, why do we care so much? “I remember being asked in Prague not so long ago, ‘What is your obsession, you Americans, with American music?’ ” said Robert Spano, the music director of the Atlanta Symphony Orchestra, which will perform at SHIFT on March 31. “The only answer I could give . . . was: It’s because we don’t know who we are, and so we’re endlessly fascinated, because there are so many things that make up America . . . so much to wrestle with and balance and try and understand. . . . I was kind of defending our self-obsession.”