Because Rhys needs the slap
One of the things that utterly shocked me about Fallout 4 was how it stops. It doesn't really end so much as it stops. Given the structure of the storylines in the main game, I was absolutely positive that a later DLC would continue the story with a conflict between the Minutemen and the Brotherhood. It seems like such an obvious perfect narrative setup to have the ideal of Personal freedom represented by the Minutemen on one side of the conflict, while having the Brotherhood represent the idea of absolute security/safety on the other side, and the remains of the Institute as a pointed reminder of the failure of either smack dab in the middle.
Well... obviously that didn't happen.
So here and there I've worked to make examples of my version of this alternate storyline. One of the results of this effort is this little 3 page comic I made showing the first meeting between General Cunningham (My version of the Player character) and Danse's team. Admit it, we all like seeing Rhys getting a verbal bitch smack.