Answer: declare the causes.
The explicit purpose of the document, as stated in these lines, is to "declare the causes" that impel different peoples to separate from each other. This is important, the author argues, because these people had been bound together for a long time, and the laws of nature and God entitles them to respect. In order to be respectful to the long connection Americans have had to England, the author wants to explain why Americans feel inclined to become independent.