Overall, the Declaration is an argumentative piece which lays out evidence supporting the assertions made therein.
In the first and rather long single-sentence paragraph, the writers locate “one people” as the central subject. They also state that it is “necessary” for this unified group to act in a distinct way. Toward the end of that sentence, they say that this necessity requires said people to “declare” the causes of their future actions. The purpose of that whole long sentence, therefore, is to set up the following sentence, which enumerates the specific factors. As they have already established that the people are acting as one, the reader has formed an idea of who the “we” are in the next sentence. That “we” is confirmed as “the governed." While the paragraph begins with a long sentence, using “that” as a lead-in multiple times establishes it as a list of the “truths” referenced in the sentence’s beginning.