I suspect they probably put them together without planning it at the start. But that's a moot point because it looks like it was planned from the start.
The slowly-developed Odo/Kira relationship is easily one of the best-developed, if not the best-developed romantic relationships ever on TV.
Worthless, standard "love interest of the week" nonsense, with which DS9 and all other Trek shows are bogged down by wasting episodes on, can in no way stand up to years-long development as is done with Odo/Kira.''
Hopefully in the future "love interest of the week" nonsense content will go extinct, like it should.