Followers romancing each other is one of the things that I've really liked BioWare doing recently.
We keep asking for more realism in our games. For more depth. For verisimilitude. We want a living, breathing world. One way of doing that is to make companion characters more three dimensional, to make it clear they have lives beyond what we see when we happen to click on them to have a conversation.
These characters don't belong to me, like pets sitting around waiting until I shout "heel." I have favourite characters, I have preferred romance interests. But I'm not so self-centred/insecure/egotistical (throwing all of those out there because I really am not sure what drives this) that I demand they have to stand around passively doing nothing even if I'm currently romancing someone else.
I want characters to turn down my advances. I want random NPCs to flirt with me, where I can flirt back, politely take the compliment, tell them I'm not interested, or admonish them for being inappropriate. I want to be able to flirt with companions and NPCs and have it lead precisely nowhere because after a while we realise to start a relationship would be a really, really bad idea. I want to make rich friendships with characters - friendships that rival romance storylines in terms of content and screen time. I want to have a proper BFF companion who I can go to when I want to discuss what just happened, and pour my heart out to, and get a hug from, and get drunk with, and fall asleep next to after laughing until 3am, without it needing to turn into a romance. I want to be able to create a playthrough where it isn't necessary to romance anyone because there is rich, rewarding friendship content that fulfills my desire for some sort of emotional attachment to the game. I want to overhear my companions talking about who they find attractive, who they don't like, who they had a rather indiscreet liaison with the night before. I want my companions to fall in love with someone not in the party. I want my companions to go through a break up and come to me for consolation.
I want all of this to be incidental stuff that makes up part of the background tapestry of the game, where it feels integrated, perfectly normal, just part of a living, breathing world I'm inhabiting.