When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation. (1)

A "decent respect to the opinions of mankind" should also require a shorter opening sentence, IOHO. This opening paragraph famously talks about what people should do when they have to break away and start their own nation. Generally speaking, of course, we haven't gotten to America yet. Jefferson just says that those people breaking away, whoever they may be, should explain themselves, implying that he will do just that in the following paragraphs.

Jefferson sets the stage for the rest of the Declaration by equating the action of these colonies declaring independence with a more grandiose human ideal. The American Revolution is within "the Course of human events" (1). It's not just a weird fringe group asking for something ridiculous, they are in fact part of a noble tradition, and they are following the proper course of action through this very document.

The next paragraph contains a lot of philosophical ideas about what humans deserve and their role in government, so the first sentence sets up that ideological discussion by instantly making the declaration into a meditation on humanity. Jefferson begins with the vaguest, broadest idea, and then gets steadily more specific, until finally we get the particular facts.

By the time the reader gets to the nitty-gritty evidence, he or she has already been given the tools to see the colonists' ideological perspective.