"El Principio Y El Fin" (The Beginning and the End) sounds like a sweet sierreño love song for most of its length, and that is the trick. She tells him there will be no tomorrow "si te falto yo" (if I am missing from your life), that he is "lo mejor para ti" (the best thing for you) and "la única razón, el principio y el fin" (the only reason, the beginning and the end). He feels "muy afortunado" (very lucky) just to have her there.
Then the floor drops out: "Si todo es tan perfecto, ¿por qué me hiciste esto?" (If everything is so perfect, why did you do this to me?). The answer is that she cheated, "¿por qué diablos me engañaste?" (why the hell did you cheat on me?). Two useful bits for learners: pa' mí is the spoken shortcut for para mí (for me), and the spoken break "pa' que se curen los raspones" (so the scrapes can heal) jokes that the music is the bandage for his heartbreak.