I just watched "Looking for Par'Mach in All the Wrong Places" (S05 E03), and something that really appeals to me about the Keiko/Miles/Kira ship is that Keiko and Miles are very much part of a family in a way that Kira's never had. Kira grew up in a refugee camp, believed her mother died when she was 3, joined the resistance when she was 12--effectively, due to the occupation, she never got to have a childhood like, say, Molly has.
While Miles has plenty of war memories of his own, and I assume that's at least somewhat affected Keiko, all of that happened when they were adults so they still had that reference of what a good childhood looks like. Miles still views Ireland as a home, and when he goes back there, he is going back to both a family and memories that are good. (I don't think we've explicitly gotten similar information about Keiko, but I'm chalking that up to her generally not being present in the show as much.)
Kira, in contrast, has never had a home in the same way. I doubt she'd go back to the refugee camp to reminisce, and she was constantly moving during her time with the resistance. When DS9 starts, Bajor has been free for less than a year, and Kira's living on a space station designed by Cardassians, serving under a Federation officer. She is proud of Bajor as her home, but she doesn't have a home on Bajor.
Furthermore, much of her relationship to Bajor itself is either based in the past ("Bajor used to have freedom/sophisticated technology/important artistic achievements/etc") or the future ("what does a healed Bajor look like; should it join the Federation; what will be its role in trade"). This is also reflected in the romantic relationships she's had so far. Vedek Bariel represented the ancient religious traditions (albeit in a more progressive way). Shakaar represents the forward march of the government, especially with how his political career would go against his caste pre-occupation, which is touched upon in S04E17 "Accession."
All in all, Kira's view on her life is rarely ever in the present, which is a fantastic contrast to the O'Briens. They have a daughter who is young enough that every few months she has a new development; they regularly make a point to be romantically (and sexually) close to one another; while Keiko is far from thrilled about their location, they intentionally make it work where they are. Their jobs also contribute to this. We mostly see the DS9 crew resolving crises, but dialogue implies that a lot of Miles' time is actually spent repairing small issues when they pop up, and Keiko goes on a whole expedition to study flora, which is governed by the seasons.
Kira carrying their baby entangles her in that same mindset. Her being pregnant demands that she pay attention to where she is then and there--she's always physically reminded of her condition, which keeps her locked in a fairly narrow timeframe. Plus, her moving in with the O'Briens means that she must get used to their daily rhythms. Kira's self-proclaimed lack of imagination impacts this as well. I love it as a character trait, but it raises the question of why she's that way.
My theory is that some of it is innate, but it was exacerbated by the trauma she experienced as a child. Childhood is often characterized as a time of great imagination and creative exploration, and Kira just... never had that. I can't find the precise quote, but when she is asked (I believe by Sisko?) if she ever had any daydreams, she responds that she'd dream that the Cardassians would leave Bajor and let the Bajorans live. Her attitude towards the Holoprograms lines up with this as well; she sees little point in doing something that isn't actually happening. For her to find value in something, it must be real--it must be useful. She used her one fantasy to turn herself into a soldier. And, pragmatically speaking, there is very little that is useful about romance, or candlelit dinners, or banter that leads to innuendos. So what happens when she's confronted with them?
