In "Song," Rossetti believes death to be a liberating experience. Mourning over it is not necessary because it would increase misery.
As the poem's speaker, Rossetti is quite open that when she does die, the "dearest" left behind should "sing no sad songs" over her passing. She gives instruction to "plant... no roses at my head." Rossetti feels that there is an inevitability to death. Mourning over it is not necessary: "Haply I may remember, / And haply may forget." Death is a natural condition of being in the world. At the same time, Rossetti sees death as a freeing experience. When she writes that she will no longer "feel the rain" or hear the nightingale / Sing on, as if in pain," it is an indication that death will free her from the pain and hurt that is a part of the living world. Rossetti would argue that to mourn over it causes unnecessary pain when it should be seen as something liberating.
In the concluding line to another one of Rossetti's poems, "Remember," she writes that we should not "remember and be sad." "Song" conveys a similar attitude towards mourning. For Rossetti, mourning brings extended and inappropriate sadness to an experience that transcends attachment.