The Eucharist is the source and summit of the Christian faith, east and west. Therefore, it is not surprising that Orthodox Christians would feel strongly about the levened Eucharist that they have recieved since Childhood and that their Church has used from antiquity. They may feel as Cardinal Cerularius did in 1050 A.D., that no Eucharist that uses unleavened bread is a true Eucharist. He took his army into the Latin Churches in Constantinople and threw the consecrated hosts into the streets.
Most Popes of the past few hundred years, have felt that his subsequent excommunication was not the best move for east-west relations, but there is no doubt that he was in complete odds with the faith of the Early Eastern Fathers, all of whom recognized the validity of the unleavened host. Cerularius died alone and alienated from the Church, both east and west.
The reason why Catholics celebrate an unleavened Eucharist is very simple. We just never changed it from the day of the last supper. We have no problem with the Eastern tradition of levened Eucharist, and its validity, and we have no desire for the east to change. But we also have no desire to change our unleavened host.
If the unleavened Eucharist is invalid, then no Eucharist ever offered in the Western Church, or in Armenia, or the Last Supper itself, (which was a Passover feast, where only unleven bread was available) was ever a true Eucharist.
I have a full discussion with Evangelicals, on the Eucharist here.
Charis kai eirene (Grace and Peace)
Lord Jesus, let Your prayer of unity for Christians
become a reality, in Your way.
We have absolute confidence
that you can bring your people together,
we give you absolute permission to move.