Hatred of the Railroad and the BOS.
The Minutemen are a Positive Faction for the Commonwealth in the Game.
The Institute will be under a Tolerant and Progressive Leadership (on most of My characters) that wishes to prevent Morloks from using Eloi as a food supply in a distant future.
The Brotherhood would prefer to keep Humanity that is not part of the BOS on the Planet huddled around campfires and using sticks. Hand chipped spearheads are signs of EVIL TECH and must be eliminated!
The Railroad are misguided and do too little to improve the Lives of the Mortal Survivors of the Great War.
Bethesda should have brought the Followers from New Vegas, or allowed the Minutemen to become a stepping Stone to building a Standing Militia to deal with the Mutant, Raider, and Gunner problems within the Commonwealth, and to keep the Pitt from expanding East and Swamp Folk at Home.