Maybe Dorian is a romantic at heart. He’s sentimental and when he makes an attachment to a person, it’s long-lasting.
He’s not reading romance novels in the corner like Cassandra. (not any more) It’s more daydreaming and holding onto little gifts way longer than is reasonable. Maybe he casts a spell on a flower to keep it alive, or uses a love letter as a bookmark in whatever book he’s reading today.
He spent so long in Tevinter thinking of any real relationship as something completely unattainable and beyond his grasp, why not go all out with the fantasizing? Bouquets of roses and Dramatic Gestures and public declarations of love, longing glances and moonlit walks and maybe some sort of solemn, poetic promise. Maybe in front of friends.
The Iron Bull, on the other hand, has none of those expectations. It wouldn’t really cross his mind. But maybe he hits on something accidentally, a particular phrase or gesture that sticks with Dorian, and of course he notices the reaction.
He likes giving people what they want, and what they need, (and the line blurs a little now that he’s only focused on his own needs and not the Qun’s) and he really likes giving Dorian what he wants, so he encourages it a little. He gives Dorian flowers, memorizes a poem or two, takes him on picnics outside the walls of the castle.
And the first time Dorian writes Bull a note when one of them goes with the Inquisitor and the other stays in Skyhold, maybe he gets it. Maybe he overhears Dorian struggling to put together a sentence in Qunlat and grammatically, it’s total crap, but the thought gets through fine.