Dullness in people
Must you harp on the same string?
Must you keep harping on that?
Must you dwell on the subject?
Must you beat a dead horse?
Don’t be such a stick-in-the-mud.
Don’t be such a party pooper. (jocular)
a party pooper = a dull person who ruins parties
Don’t be such a wet blanket. (informal)
a wet blanket = someone who ruins all the fun, as a wet blanket smothers afire
Don’t be such a killjoy. (informal)
a killjoy = someone who ruins all the fun