I confess, I rarely write romantic stories.
I mean, I've got 400+ characters kicking around, scattered across various stories and imaginary worlds, and some of them do have relationships and romances and lost loves and whatnot - but I rarely get to put it into actual finished stories that other people see. For some reason, it just hasn't happened a lot.
That said....
I like unlikely romances. While the whole romcom love at first sight thing is sweet too, if done right, and I love to see it in other people's stories, I tend to lean more towards romances that might not have happened. I like slow burns, where the characters fall in love gradually, over a long time.
I like contrasts between the characters - not just in personality, but in appearance too. Tall people falling in love with short people, blonde people falling in love with brunettes, heavily tattooed people falling in love with someone who never has a hair out of place, etc., etc. I like weird, off-beat, kinda broken people falling in love and finding ways to put themselves back together.
I have this one couple - two guys in a kind of steampunk-influenced world, which I desperately want to do more with, but haven't gotten around to yet. One of them is a stiff-necked, very formal, by-the-book high ranking officer in the airship navy, who keeps his feelings and thoughts so close to his chest he might as well stitch them into the inside of his waistcoat. The other is a lower-ranking officer in the same navy, who is very informal, open and honest and full of bubbling enthusiasm and brilliant ideas - most of them about building airships. He talks a mile a minute and keeps airship blueprints tucked away in his uniform sleeves. They are each other's opposites in nearly every way, and fall in love slowly but inevitably - kind of like an avalanche, but with less casualties.
They work as a couple because despite their differences - differences they appreciate, by the way; the by-the-book guy likes the bubbling enthusiasm of the lower-ranking officer, the lower-ranking officer likes how the higher-ranking officer is serious and dependable and never backs down from trouble - they both value the same things, and work towards the same goals. They trust each other.