I always took it that the symbiont could hide certain things from the host and, as shown in season three, even itself if the situation warranted it. Maybe knowing that Curzon was in love with her would have been a strain on Jadzia or caused a problem to the symbiont (similar to why Joran's history had been eliminated from Dax).
In that case, Dax and Kahn should have hidden their love for each other (or at least the degree of intensity of that love), given the threat such intense feelings can pose to the lives of the symbionts. Why would the Dax symbiont hide Curzonís love, which was kind of awkward, but not Toriasís love, which could threaten Daxís continued existence?
In many ways the situations were entirely different.
This could have been a strain on the symbiont just as much, but it was different because the symbiont itself may not have been involved with the feelings being felt. Curzon himself seemed to be in love with Jadzia, not Dax as being in love with her.
I see this more along the lines of they'd spent so much time together that the symbionts had fallen in love with each other just the same way that the people had. When Torias died and Dax moved on it was a strain on the symbionts and being put back together caused old feelings to resurface that were dangerous to the symbiont.