I assume she doesn't want to just write "I love you" because some things can only be properly conveyed vocally. Same reason you'd leave a letter telling someone to come some place for a confession instead of just leaving a love letter in their shoebox, as the old trope goes.
I was sorta doing it unconsciously before I think, but now I’ve actually consciously realised my brain is automatically giving bear head Tomokazu Sugita’s voice.