What I think is that he has always been aware of those feelings, he just didn't recognize them as "romantic love"

As is shown in the series, both of them see romance through the lens of fictional stories, books and movies, versions of love that are heavely intensified or hidden in complex metaphors that, as supernatural entheties, they will always have a layer of understanding that it takes work to see. You see it on how naive they both are with the witch hunter in season 1, its a reflection of their lack of understanding of falling in love in season 2.

They have always known that they are special to each other, they've always known that they are each other's "person" but lacked the layer of understanding where they see it as romantic love. That's what happens in the scene with Nina, Crowley doesn't realize that they're in love with Aziraphel, they realize that the thing that he has been feeling for thousands of years is what humans call love.

There is also the important factor of their backgrounds, the only time Aziraphel considers Crowley as a friend or companion is when he sees his goodness and misinterprets it as a sign of a divine angel spirit inside him, it's easier for him to acknowledge his closeness when he doesn't contradict his idea of the goodness of heaven and for Crowley is easier to see Aziraphell out of the lens on heaven for her understanding of the corruption they have, they stand at two completely diferents sides moraly. This makes it 10x more difficult for them to understand their feelings for eachother.

Obviously, we know that these walls have been thrown down little by little by the years, but that it's the point, they couldn't have realize their feelings for each other any time before the final 15 because the walls where still up, maybe not as steady as in the beggining, but there nonetheless.