In Musaf of Rosh Chodesh we say, "Mizbeiach Chadash B'Tzion Tachin U'BiAvodas Bais HaMikdash Nismach Kulanu", a new Mizbei'ach should be built in Tzion and we will all rejoice in the avodah of the Bais HaMikdash. Why does it need to say we will "all" rejoice, of course we will all rejoice?
Rav Michel Stern in Iyun Tefila brings that at the dedication of the second Bias HaMikdash the young people rejoiced, while the old Kohanim cried. The young ones were delighted with their Bais HaMikdash but the older ones who remembered the first Bais HaMikdash cried over what this new Bais HaMikdash lacked that the first one had.
In our tefila during Musaf we say that we will all rejoice because the third Bais HaMikdash will lack nothing and will bring joy to all of Klal Yisroel whose celebration and simcha will know no bounds Bimheira BiYameinu Amein.