Yet -- and this may be a sign of some sort of pathology on my part -- reading her poetry never fails to cheer me up. The same thing happens when I read Philip Larkin, another poet who has a reputation (deserved or not) for glumness. (Larkin, by the way, was a great admirer of Rossetti's poetry.)
Something that John Bayley wrote is perhaps pertinent: "happiness is not common in good poetry, nor of much value to it." John Bayley, "Spruce" (review of A. E. Housman: Collected Poems and Selected Prose), London Review of Books (June 2, 1988).
Here is Rossetti's and Larkin's secret: their poetry is lovely. I realize that "lovely" is not exactly a rigorous critical assessment, but it is the best that I can do.
"A Study, in March" (c. 1855)
From Sunset to Star Rise
Go from me, summer friends, and tarry not:
I am no summer friend, but wintry cold,
A silly sheep benighted from the fold,
A sluggard with a thorn-choked garden plot.
Take counsel, sever from my lot your lot,
Dwell in your pleasant places, hoard your gold;
Lest you with me should shiver on the wold,
Athirst and hungering on a barren spot.
For I have hedged me with a thorny hedge,
I live alone, I look to die alone:
Yet sometimes when a wind sighs through the sedge
Ghosts of my buried years and friends come back,
My heart goes sighing after swallows flown
On sometime summer's unreturning track.
Christina Rossetti, Goblin Market, The Prince's Progress and Other Poems (1875).
So, where lies the beauty that dispels the gloom? I'd say exactly here: "On sometime summer's unreturning track."