The Royal mottos or valgspråk/valspråk of the Norwegian monarchs are an old tradition, permanent since the reign of Christian II of Denmark and Norway. The latest three kings have adopted the same motto as their personal motto, Alt for Norge, that has a particularly high standing as it became one of the main symbols of the Norwegian struggle during the German occupation of Norway in World War II.
Note: (1) Compare with the motto of the U.S. state of Missouri (1821): "Let the welfare of the people be the supreme law". Originally from Cicero's De Legibus, Book III, Part III, Sub. VIII. (2) The present royal motto is not easily translated into English: the literal meaning is "All for Norway" but this conveys a sense of "Everything for Norway" as opposed "Everybody for Norway" (since "alt" is in neuter gender, and would rarely refer to people). As such, it conveys a meaning of "Give all for Norway", which is interpreted as the readiness (of the sovereign, but maybe also his subjects) to give up or sacrifice everything for Norway.