Lovely word, brooding. It's one of those words that seems to enact itself in its very sound.
Odd to think of the connection between the relatively neutral notion of a brood as a group - a brood of hens - and the negative connotations of referring to your chum's family as a brood. Does the sarcasm of the latter derive from the sense of morbidity of thought when you find yourself brooding over something?
It appears to stretch back to an Indo-European base related to burning or heating - hence the incubation of eggs, and according to one authority, John Ayto in The Bloomsbury Dictionary of Word Origins, the sense of worrying associated with brooding only developed in the eighteenth century. But that doesn't sound right to me. It's easy to see how the link to becoming over-heated in thought was likely to have been there from the very beginning.
I reckon it's a very late-nineteenth century sort of word. All those Romantic, fin de siècle types seem to have been brooding all the time. Yeats's line(s), and brood / Upon love's bitter mystery, has been stuck in my brooding mind all afternoon (via Ulysses, I think, in which Stephen D. quotes Yeats's great song and does some substantial brooding himself. But Bloom doesn't really brood, does he, and neither does Molly. Gosh, wasn't Joyce amazingly good at capturing the textures of different modes of thought?) Oh, and I've just realised apropos Yeats's poem that he uses the word twice, in adjacent lines, no less!
Of course, healthy minded folks like myself get beyond brooding, becoming sensibly reflective. Or so we like to imagine.