The saddest thing about love, Joe, is that not only the love cannot last forever, but even the heartbreak is soon forgotten.

The speaker laments a double impermanence: love fades, and even the pain of losing it does not endure. The line suggests that what feels most defining—passion and its aftermath—proves transient under time’s erosion, leaving a person not only bereft of love but also of the very sorrow that once testified to its depth. Addressing “Joe” frames the thought as intimate counsel, almost a hard-earned lesson: memory is unreliable, and emotional experience is not preserved in proportion to its intensity. The sadness, then, is existential—nothing, not even heartbreak, grants lasting meaning or permanence.