And why not? No need to think up a wholly new plot, no need to write much (any) dialog. As with the line above, improv permits the characters to ad lib generously and when they aren't dumbing down the dialog they can scream, jump around, and act stupidly like adolescents on a tough voyage into manhood.
Substitute Bangkok for Las Vegas, as our chums pretty-boy Phil (Bradley Cooper) and odd duck with blank stare Alan (Zach Galifianakis) celebrate the approaching wedding of formerly married Stu (Ed Helms), with teen brother-in-law-to-be (who disappears for most of the film) accompanying them. Substitute the critters from the first film with a mostly computer animated monkey and you have the duplicated plot, replete with car chases, wild dashes in and out of unsavory places, contact with still more unsavory characters - all trying to reconstruct what went on to lead up to the hangover and its consequences.
Why waste time on this sequel; you'll get the same entertainment (if that's the kind that suits you) from the original, just watch it again. Director Todd Phillips, who also worked on the script (There was a script?), does his best in recreating the whacked-out energy, but without the element of surprise, it's a case of 105-minutes of been there, done that. (Grade: D+)