Why is no one save Hamlet upset at the marriage of Claudius and Gertrude?
I have only just started the book and may not yet have encountered others who do object to their union, but as far as I can see nobody seems to mind or be suspicious that this has happened so soon after King Hamlet's sudden death.
Hamlet is very much upset by the hasty marriage of Claudius and Gertrude because he is personally affected in more than one way. He was the heir apparent, but Claudius took advantage of his absence to make deals to get himself elected king, and by marrying the queen he solidified his position. Claudius publicly proclaims that Hamlet will succeed him, but it seems possible that Gertrude might have another child, and Claudius would certainly favor his own son or daughter over his stepston. And Hamlet strongly disaapproves of his mother's behavior on moral grounds. Nobody else is severely affected by the marriage of Claudius and Gertrude--and if they were, they wouldn't dare to express their thoughts openly for fear of getting their heads chopped off. When Hamlet says, "I pray thee, do not mock me, fellow-student. I think it was to see my mother's wedding," Horatio discreetly replies, "Indeed my lord, it followed hard upon [his father's funeral]." This very guarded comment is the strongest expression of disapproval by an outsider that you will find in the whole play