Had you started dating him at that point? If so, he's thinking that two people he cared about got together behind his back and betrayed him. It's not cool to be hooking up with friends of the person you're seeing.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy