THE LAWS OF KADDISH
There are certain parts of davening that are called "a matter of Kedusha" (holiness). Prayers of this type can only be said when there is a minyan. Saying Kaddish with a minyan is a way of making a big "Kiddush Hashem" (a sanctification of Hashem's name). A person should say "Amayn Y'hay Sh'may Rabboh" with much concentration and love. It should also be said loudly to help him concentrate. This can cancel bad decrees.