Most Helpful Guy
Tribalism's a thing. Most people are subject to it and don't like to see members of what they perceive as "their" group interacting with others. It incurs feelings of dishonesty and disloyalty. It's a visceral reaction, and most people can't help it.
You might not be able to do anything about what you feel, but you CAN recognize why you feel that way and choose how you're going to act on it.