Because it hasn’t always been bad.
For example, in Tunisia during the 19th century mixed faith communities would make pacts of protection in case of conflicts or feuds:
In times of danger [...] each Jewish family had a Muslim guarantor to whom it would turn; this person was supposed to protect them from injury and to avenge them if they were harmed.*
How strange, that we seem to be going backward in terms of societal prejudice and persecution, rather than forward.
Our ancestors may have got many things wrong, but some things wonderfully right.
Like communal cohesion.
* Meyers, Allan R. ‘Patronage and Protection: The Status of Jews in a Berber Environment’ in Jewish Societies in the Middle East: Community, Culture and Authority. New York: University Press of America. 1982. pp. 85-105.
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Not sure about the avenge part, but the attempt to build trust with individuals in the “other” faith community is laudable.
Eric Beinhocker has written a book entitled “The Origin of Wealth” in which he charts the relationship between trust and economic performance (p.433)(based on year 2000 data).
Improving the level of trust is a necessary part of building peace. Keeping appointments and promises, repaying debts, acknowledging past mistakes, recompense, selflessly motivated offers of help or assistance — perhaps things like this can help to start heal the wounds.