In act 1, scene 2, Claudius addresses his court and comments on his mixed feelings of happiness and grief. While he is happy to celebrate his recent marriage to Gertrude, he is apparently unhappy about his brother's tragic death. After Claudius gives Laertes permission to return to France and attend school, he encourages Hamlet to cheer up and stop mourning his father's death. Claudius is aware that he is in a precarious position after usurping the throne by assassinating his brother and wants everything in his court to appear stable and upright. Claudius also understands Gertrude's love for her son and does not want Prince Hamlet leaving the country, where he can organize forces and stir up a rebellion. When Claudius requests that Hamlet stay in Denmark instead of returning to Wittenberg, Hamlet agrees, which is good news for Claudius. Claudius's plan is coming together and he is happy that Hamlet agrees to stay in the country, which is why he says,
Why, ’tis a loving and a fair reply. Be as ourself in Denmark.—Madam, come. This gentle and unforced accord of Hamlet Sits smiling to my heart, in grace whereof No jocund health that Denmark drinks today But the great cannon to the clouds shall tell, And the king’s rouse the heavens shall bruit again, Respeaking earthly thunder. (Shakespeare, 1.2.121-128)
While Claudius is indeed celebrating his marriage to Gertrude, he is also happy that Hamlet chose to remain in Denmark, where he can keep an eye on him and make sure that he does not have any plans of causing a rebellion. Claudius also plans on celebrating by drinking copious amounts of alcohol with the royal court.