[Avodah] deprive the minyan of the opportunity to say tachanun

[Cantor Wolberg]

Regarding this very topic, I heard a beautiful vort by I believe 
Rabbi Moshe Kesselman. He taught that when one Jew has 
suffered a loss, then it affects all Jews since we should be as one.
In the same vein, conversely, when one Jew celebrates a simcha,
we should all celebrate and feel the joy (not necessarily in a literal sense
but figuratively speaking. This is true achdut and therefore, if there is a 
chatan at the minyan, we purposely don’t say tachanun because we feel
his joy.