I used to know so long ago;
Summer has followed after Spring;
Now Autumn is so shrunk and sere,
I scarcely think a sadder thing
Can be the Winter of my year.
Yet Robin sings through Winter's rest,
When bushes put their berries on;
While they their ruddy jewels don,
He sings out of a ruddy breast;
The hips and haws and ruddy breast
Make one spot warm where snowflakes lie
They break and cheer the unlovely rest
Of Winter's pause--and why not I?
The poem ends with an unanswerable  rhetorical question . The poet asks “why not I?” — an ambiguous question. Does she think that she will be cheered, or is she saying that she doubts if she will  — as if she senses happiness will be denied her.  So the poem ends without resolution. 
 On a biographical note, this poem can be viewed as encapsulating two elements of Rossetti’s life — her positivity in her younger years and the depression and illness that blighted her later life.
The Key-Note