How did Roger Chillingworth and Mr. Dimmesdale become such good friends?

I think the operative word here comes from the title of this chapter! Dimmesdale isn't in good health. Chillingworth presents himself as a doctor and the two men become room mates. Really Chillingworth is like a "leech" slowly sucking the life out of Dimmesdale but the superficial pretence is that they are "friends".