Why can't I write a simple fluff piece?

Back in 2016 over on (I think) the bunny farm on LJ, someone by the name of Bamshee asked for the following fic: "Fav trope: Severely traumatized Character A waking from a nightmare in Character B’s arms. Character B consoling and comforting panicky and upset Character A. Character A clinging to Character B. Character B cuddles Character A through a long, sleepless night." I pretty much fell in love with the prompt and started writing it with Smokescreen having the nightmare and Ratchet comforting him. And then the dreaded writer's block hit and I was left with a few lines and not much else. Well, I have started writing the fic again, but instead of a nice, soft little fluff piece, I now have:Smokescreen with a PTSD induced panic attack, him refusing to talk about it, Ratchet offering support, Smokescreen trying to break up with Ratchet rather than talk, and now Ratchet worrying that Smokey's going to do something very very stupid as a result of said PTSD.

I just want to write a soft little fluff piece! Why is it insisting on turning into a damned darkfic with horrible rapey undertones?! I now have 5 pages of nothing but pain and angst .... One day I will be able to get back to nice fluff where I don't torture my characters. I just have to get all of these dark little bunnies out of my headspace.

EDIT: Of course I'm not sure if I'm even writing a PTSD panic attack properly of if this is just going to come across as unnecessarily cruel and vastly wavering in consistent characterisation. ... Maybe I just need to stop second guessing myself.