His Shrek is, indeed, the perfect foil for the Donkey's irrepressibility.
It is rendered the more poignant by our knowledge -- not, of course, shared by the characters -- of how brief and repressible their irrepressibility would prove to be.
But his work in theater (he previously did the Rocky Horror Picture Show set) so showcases his "Let's put on a show!" irrepressibility that it calls for an encore.