It's going all the way back 1878 Ottoman - Russian war. Ottoman Empire residents Armenians took side of the Russian Empire because of the religious issues, and rebelled against Ottoman Empire. That caused Caucasia to divide, and Armenia was founded. Many migrations took place at that time, many people driven from their homes from Caucasia and come to Anatolia, especially to Black Sea region.
During the first world war, Armenians living in Ottoman soil rebelled again to unite with Armenia and slaughtered many Turkish citizens. Many villages were burned. Even babies were burned alive. Despite of being busy with the war taking place in the west, Ottoman Empire sent a formidable force to East Anatolia to end this slaughter. The army defeated all the Armenian militia, and Ottoman Empire exiled Armenians from their soil to south and grand migration took place.
It's estimated 3 million Turkish people was killed during the Armenian rebellion, and 1,5 million Armenians was dead because of the war, conditions during the migration such as cold, starvation and plague.
To many deaths from both sides, that is the main reason for the hatred. Ottoman Empire never did "genocide", though starvation and plague did.
I wonder what would be the reaction of the USA if 3 million of their own people was slaughtered? Exiling was the most human punishment at the time.
Edit: FYI My Grand grand father's brother was skinned alive at a town near Erzurum.