When you're dwelling on something, you're brooding about it — this can be thoughtful or morbidly obsessed.
When you're brooding, you might be depressed about something you just can't stop thinking about — like the lead character in Hamlet. Brooding can also mean you're being extremely thoughtful, contemplative, meditative, musing, reflective, or ruminative — those are all good things. You have to look at the context to see if someone is brooding in a positive or negative way. This is also a word for what chickens and other egg-laying animals do: sitting on eggs to help them hatch.