curistar

I've never written any MLP stuff, but I've written in the romance genre for over 10 years now. (I feel so old typing that!) For a romance to develop naturally I also suspect you mean gradually. As the characters get to know each other they will discover they have more and more things in common - they can be large things like the same hobby ("We both love to read"), small things ("I love apples, too!"), or even emotional things, noticing that one person reacts similarly in a given situation ("I would've gotten angry, too.") That's usually what sparks the initial compatibility. The more time they spend together, the more they open up to one another. It cumulates when they realize they want to help solve each other's problems, because they want to help the person they care about. And when it comes to light for one of them (or maybe both of them at the same time) that they care about that other person more than themselves, well, there you go, you've got a romance. Hope this helps!