That's a glitch. Something Bethesda never intended to be used and would have patched out if they weren't too busy being lazy and incompetent spazzes but keep telling yourself otherwise. Now to bring up how New Vegas built up companion affinity because I feel like it (LOOOOOOOOOONG post incoming): Craig Boone starts out not trusting the Courier a lot even after completing his quest since he has trust issues. Only after assisting him in wiping out major Legion camps, rescuing slaves and more would he open up slowly to reveal his actual problems that have plagued him after leaving First Recon since the Courier had fought alongside him, listened to him and given support to him throughout these instances. Veronica Santangelo starts out friendly but does not talk about her current issues and dilemma to some random stranger she's following. By bringing to places where advanced technology is being used by the other factions in the Mojave will she open up about these issues and her current dilemma since she slowly makes up her mind about her own conflicting views about the current Brotherhood. Arcade Gannon can be convinced to join you and from his intellectual comments, he appears to be a highly knowledgeable member of the Followers but when the Courier takes him to locations with a certain type of advanced tech, he starts to slip up on where that knowledge comes from. Getting him to trust the Courier by performing actions he agrees with (or convincing him how the Courier's current actions align with the well-being of the Wasteland) will eventually build enough trust to convince him to reveal his secrets to the Courier and whether or not he should live up to his legacy. Raul Tejada starts out as a sarcastic and aged companion but has his own issues and doubts that he does not feel like talking about to someone who he just met even if said person saved his life and all. By bringing him to other elder folks who are still working past their prime, he begins to question himself and wonders what he should be doing next which the Courier can answer to help Raul with his issues and doubts about being of service despite his advanced age. Lily starts as a sweet though intimidating Super Mutant who treats the Courier like her old grandchild due to the onset of Schizophrenia from being a Nightkin and age affecting her. By learning about her, the Courier is able to consider the type of person she is and eventually help her decide on the best course of action for her when the matter about her medicine dosages comes up. (This one had more impact on me because upon hearing that tape and how much her grandkids meant to her, I could decide immediately even after my one hour debate on what to do when I got to this point) Rose of Sharon Cassidy is a smart-mouthed caravan owner down on her luck but starts out friendly (plus the skill-checks allows the Courier to build a rapport with her better). There was no need for a quest to build up friendship with her since the Courier would have built a rapport with her by negotiating for the sale of her defunct caravan company (or beating her in a drinking contest) but the Courier can help her settle her lingering doubts about what happened to her ransacked caravan. Rex and ED-E do not have any affinity building but this is because they don't need such things; one is an Eyebot that is programmed to be loyal (though Lonesome Road changes this to an extent) while the other is a dog told by his previous master to be loyal to the Courier. They make up for this by having plenty of history and backstory to them. Spoiler: Some spoilers on outcomes for companions One thing in common for all this is the differing outcomes for these companion story-lines and how different these companions are due to the outcomes. Boone can be made suicidal but more deadly through guilt or be convinced that he is still capable of redeeming himself through atonement. This changed mindset even affects his ending and changes based on who took over New Vegas in the end. Veronica can choose to plug her ears to the failings of the Brotherhood to stay with her family or abandon them to become a wanderer free from the Brotherhood while anonymously aiding others in handling tech. Arcade can embrace his legacy even with the implications of being a known Enclave Remnant or choose to turn his back on it to live out the rest of life free of his past as a doctor. Raul can decide to retire in peace as a simple mechanic after everything he had gone through or reinvigorate him so that he could embrace who he was before and continue using his still-present skills as a heroic gunslinger. Lily can be convinced to stop taking the medicine entirely (at the cost of allowing her savage nature to take dominant control over her), take the full dosage (which cures her but erases her old memories) or maintain the half dosages (which keeps her the same as before but prevents her from being cured). Cass can enact bloody revenge on the Van Graffs and the Crimson Caravan with the Courier's help or be convinced to destroy the two groups through legal methods. Both options grant her a different perk to reflect her mindset by the end of her quest-line. As for Rex and ED-E, the former can be given a new brain that affects the kind of personality Rex may have by the end of the Courier's story while the latter determines which group obtains the technology housed in ED-E. TL,DR; The companion affinity of New Vegas worked by being consistent and feasible in terms of writing and mechanic integration. It was built up by giving the characters motivations and ideals that the Courier could agree with in-game through dialogue and actions. From what I can tell by slogging through FO4, FO4 tried to do something similar but they ignored the writing and complexities involved in New Vegas which led to the whole lockpick a door or use drugs etc system to gain approval. It was a stupidly dumbed down system after New Vegas that reflects Bethesda's approach to Fallout games: To never learn from the mistakes of the past or never truly improving from good ideas already implemented by other content makers.