Anger is a cover of pain. When people cross our boundaries, we should focus on expressing our pain instead of anger.
Anger is not a base emotion but a secondary emotion that's often triggered by pain (though some may disagree). When the pain caused by someone exceeds the maximum threshold we can endure, that pain turns into anger. Anger is, therefore, a warning sign that our boundaries have been crossed.
Instead of expressing anger, it is more effective to focus on the underlying pain and communicate how our boundaries have been violated. Expressing pain and its impact in a non-accusatory manner can lead to better results than expressing anger, which can cause people to become defensive and unresponsive.
For instance, we can say things like, "I wanted to talk to you about a boundary I have and how it was crossed. Here's how it made me feel. If you're interested, I'd appreciate it if you could do things differently in the future". When we share your pain and the boundary that got crossed, we're never accusing. We're just saying, “This is what’s going on with me”, and people are more open to hearing that.