Seth Rogen has played in numerous comedies, but notably, only has one role. He plays an underdog that every man can relate to.
Seth Rogen owes Judd Apatow a blowjob, at the very least. He probably owes Judd Apatow's wife, Leslie Mann, whatever she desires. Judd Apatow looks like this:
Leslie Mann looks like this:
And that is why he was able to cast someone as universally unappealing to the ladies as Seth Rogen in 'Knocked Up".
Seth Rogen also owes Judd Apatow for his first (and every subsequent) break. He was cast as Ken Miller in 1999-2000's Freaks and Geeks, an off-beat comedy about the, wait for it, underdogs at a 1980's high school. Rogen actually dropped out of school in Canada to move to LA for the series. That's right. He dropped out of high school to play a high school student on a tv show. The show was cancelled after one season.
He's had a good run for the last couple of years, but things are likely coming to an end for Rogen, who's biggest chance right now is his revival as a fat, pot smoking green hornet.
Reboots are always successful.