Judd Apatow is guiding us through the stages of middle-class American life. In Freaks and Geeks, he gave us the fringes of high school, and in Undeclared, we saw college life for the typical non-participant. The 40 Year Old Virgin showed us about the introduction to sex and relationships (and a lot else), and now Knocked Up shows us the beginnings of parenthood. And since to a large extent you're watching the same actors work through these different stages of life, you really feel like you are just following life through its archetypal stages.
There's a certain realism to much of Apatow's comic work: even when the plot takes a conventional story arch, we go through that arch in a very authentic, uncontrived way. There's regular (and regular looking) people making their way the regular parts of life--but with an intensely funny edge.