Novel help: I need a reason for there to be an on-going feud between two groups

[eInspireeMee]

of guys? In my story I have two groups of guys from two different sides of town one set are upper class and one set are middle class and they hate each other but I need a good reason for them to hate each other.

Give me an idea for something that sparked the feud between the groups?

I was thinking merely the differences in their economic status that makes them hate each other but that is kind of shallow. Any other ideas?

 

[DerSchultheiss]

Hate is shallow, but it can cut deep into feelings. If that makes sense.