Actually, Squid, 1204 was just the FIRST betrayal of the Christian East by the West, there have been many betrayals since then. And each time, Eastern Christians have naively gone into it with faith that their Western brothers will stand by them and each time, they have gotten sold out.
Today's Christians in Kosovo and the Christians in Iraq, are just two of the most recent examples of this kind of betrayal. The WWII genocide of Serbs, Jews and Gypsies are another one. The genocide and ethnic cleansing of Greeks from Asia Minor and from Cyprus, with the acceptance (enforcement) of Western Powers. One could go on and on.
Point is that this isn't just "an 800 year old grudge". Eastern Christians have been the traditional buffer between the West and Islam, and that was a role that they accepted as their fate. But what they could not and cannot accept is the idea that they are being used as nothing more than "pawns" and "bargaining chips" to the West, to be routinely sacrificed for Western economic interests while we in the West still proclaim we are "doing God's Work" -- the hypocrisy of that mindset is just too much to bear without bitterness.
There were also the power plays by Frankish popes and their allies (notably Charlemagne and William the Conquerer) that led to the formation of a separate “Roman Catholic Church” in the first place, the suppression of the Orthodox West, and the split with the East in 1054. 1204 was the final exclamation point to that split.
The Lutheran Reformation (Reformation Day is Oct. 31) was an attempt to overcome the abuses of the Frankish popes, and arguably to move back toward Orthodoxy. The second generation of Lutheran reformers did try to make contact with the Patriarch of Constantinople. However, in the absence of contact with Orthodox communities, and with the Turk and the “Holy Roman Empire” in the way, that did not and could not go far enough.
The suppression of the Orthodox West led to the conflicts between the West and the Orthodox East.