I headcanon that Rufus & Cloud start off as casual sex (of the sparky, "I want to punch him with my mouth" type) that gradually softens & turns into a romantic relationship when Cloud isn't paying attention. He wakes up one day, realizes they've been dating for like six months and he's falling in actual love, and has a minor freak-out about it. (It's fine, Rufus had been expecting something of the sort, given how dense Cloud can be sometimes.)
I think what does it for Cloud is that Rufus has seen the worst of him - the experiments, the Crater - and still wants him anyway. That Rufus doesn't pity him or try to make excuses. That Rufus sees him and accepts him as he is, even the broken parts, and doesn't try to fix him. He doesn't ask for more than Cloud can give.
Also that Rufus knows who he *himself* is, what he wants, and how he's going to get it. I think Cloud would find that kind of certainty attractive! And Rufus has a life outside of Cloud, so Cloud doesn't have to worry about being made the center of everything: Rufus makes time for him, and Rufus only does that when he thinks something is important so clearly Cloud is important to him too, but (post-game) Rufus isn't going to demand every second of Cloud's time & attention.
Your headcanon is just so GOOD!!!! I got goosebumps while I was reading it. Our ship is so lucky to have you 🥺❤
I got some inspiration to draw this from reading your headcanon 😊
Cloud is indeed quite dense when it comes to romance 😂😂😂