Why did Denmark lose Norway in the Treaty of Kiel?

Denmark lost Norway in the Treaty of Kiel (January 14, 1814) primarily as a consequence of its alliance with Napoleon I during the Napoleonic Wars.2

Following the defeat of Napoleon at the Battle of Leipzig in 1813, Sweden—which had become an ally of Great Britain—invaded Denmark from the south through [Schleswig-Holstein]. Faced with military defeat and the collapse of its position, Denmark was forced to cede Norway to the Swedish king to end hostilities.123

This loss served as partial compensation for Sweden, which had previously lost Finland to Russia in 1809. While Denmark retained its rule over Greenland, Iceland, and the Faroe Islands, the cession of Norway ended a union that had existed since 1380 and significantly diminished Denmark's status as a European power.32