Like waving a red flag in front of a bull American phrase
It can also be phrased as "like a red rag to a bull" in the UK.
Provoke the anger of someone
When I mentioned my husband's business failure, this was like waving a red flag in front of a bull
Joking about her height is like waving a red flag in front of a bull.
To complain that something is unfair or illegal.
Suddenly become crazy or very angry or lose control
To provoke a person, group, nation, ect.
Used to express your displeasure
It is easy to guess that the origin of this phrase was from bullfighting, but the first animal known to be susceptible to rag-waving was not bulls as we thought.
The waving of a cloth rag at an animal to distract it may have been a common practice for centuries, but it wasn't until the 1700s that it was documented in print. And surprisingly, the first creature to respond to this was that most dim-witted of birds, the pheasant. It wasn't until 1873 that someone decided that bulls were to be added to the list, when Charlotte Yonge included an allusion in the novel Pillars of the House:
"Jack will do for himself if he tells Wilmet her eyes are violet; it is like a red rag to a bull." (Source: phrase.org.uk)
Friendships will develop well and be successful when the friends respect each other's privacy.
You should respect your friend's privacy. A hedge between keeps friendship green.