IF JOINED TRILLS ARE SUPPOSED TO HAVE THE MEMORIES OF THEIR PREVIOUS HOSTS, WHY WAS JADZIA UNAWARE THAT CURZON HAD HAD FEELINGS FOR HER?
The Situation:
In the DS9 episode "Facets," Jadzia Dax undergoes the zhian'tara, a ceremony that separates the memories/personalities of Dax's previous hosts and places them temporarily in some other person, so that the two can talk "one-on-one." The big revelation in "Facets" is that Curzon Dax, who was the person in charge of assessing Jadzia's worthiness for a Joining, flunked Jadzia from the program deliberately because he was very attracted to her, only to reinstate her later out of guilt.
The puzzle here, as it says above, if (as has been stated many times) joined Trills share the memories of previous hosts, why didn't Jadzia--who obviously has access to Curzon's memories--know about what Curzon did?
The Non-Canon answer:
Well, we know that previous hosts' memories can be repressed...the experience of an entire host was shown to be repressed in "Equilibrium." So the "how" is covered. As to why, I would make the conjecture that it's common policy for the Commission to supress memories dealing with the previous interaction of two common hosts, like Jadzia and Curzon. Think of it--what if you could remember a relationship between you and someone else from both perspectives? It could get very confusing, and may contribute to a condition--a "somatic dissonance"--whose psychological effects (whatever they are--I'm sure the Symbiosis Commission knows) can be damaging to host and symbiont. So when Jadzia was joined with Dax, she was given a treatment that suppressed Curzon's memories during the period of overlap when she interacted with Curzon--maybe not all the memories, but enough that she was never fully aware of the things that Curzon finally revealed during the zhian'tara. Jadzia was probably aware of this process and its necessity.