Some people come to weddings and don't dance or barely dance.
Why not?
It is a mitzva. Every man in shul wears tfillin b/c it is a mitzva. It is not a "if you feel like it" type of thing but a mitzva. So why are some people not dancing? I am not referring to the old or infirm but to the able bodied.
Maybe b/c people don't see it as a mitzva but something you do if you enjoy it and feel like it.
But it is not so. It is a bona fide mitzva - no less than wearing tfillin or other mitzvos. So if you are at a wedding - dance as much as you can. This is the ONLY time that this couple will ever get married. Maybe you have been to dozens of weddings but for this couple it is the first and last. Maybe you are not related and don't feel so close to the Ba'alei Simcha. No matter. We are not commanded to take part in the joy of others b/c we "feel" connected but b/c it is the right thing to do.
