Dorothea and Lydgate have some character traits in common, in that both of them genuinely wish to help others.  Lydgate is handsome, of high social status, but still idealistic enought to want to be a country doctor and change the world.  His motives aren't completely selfless, but he is able to bring reforms to his medical practice, open a new hospital, and establish a reputation as a quality physician with a respectable, and monied clientele.  Things are going well for Tertius Lydgate until he meets a woman named Rosamond, and everything goes down hill from there.  His expectations and what married life with Rosamond turn out to be two very different things.  Similarly, Dorothea is eager to help, a natural philanthropist, even, redesigning homes for local farmers, and donating a considerable amount of cash both to Lydgate's hospital and to help Lydgate himself pay off a massive debt.  Dorothea has a less-than-ideal marriage, although she doesn't realize it until her first husband passes away, when she figures out the he didn't trush her fidelity.  Her second marriage, however, becomes a source of happiness, satisfaction and character growth for her, in contrast to Lydgate's disastrous nuptials.