Yeats’s short lyric turns on a quiet, almost resigned realization. The speaker recalls that one person had a lovely face, and two or three others had charm, yet none of their appeal mattered. Those attractions were “in vain” because memory works like mountain grass: once a hare has lain there, the grass cannot help but keep the outline of its body. In other words, an earlier love or experience has left an imprint too deep to be erased or replaced by later beauty or charm.
The mood is wistful and a little melancholy, but not angry. There is acceptance in the image of grass holding a shape. It suggests that the past remains present in the mind, gently but stubbornly, no matter who else comes along. The comparison is tender and slightly eerie — the speaker does not say he chooses to remember; he simply cannot do otherwise.