Neruda isn't simply mourning that people stop loving—he's suggesting that love's brevity isn't the tragedy; forgetting is. There's a quietly devastating suggestion here that we're built to move on, that time erodes even our most vivid feelings whether we wish it to or not. When you've ended a relationship and catch yourself struggling to recall precisely how their voice sounded, or realizing months have passed without thinking of them, you understand what he means: the heart's forgetfulness is its own kind of cruelty, indifferent to how much the love once mattered.