Reassociation was the resumption of a romantic relationship between at least one joined Trill and a person with whom the symbiont's previous host was romantically involved. Platonic friendships without a romantic element were not subject to this rule and could be resumed, and in some cases were even encouraged. Since the point of each successive host was for the symbiont to acquire new lifetimes of experiences, Trills considered reconnecting with intimate relationships from a past life "unnatural." Joined Trills who reassociated were exiled from Trill society, which also meant that their symbionts died with that host, since they no longer had access to new hosts. This was a dire consequence, since for Trills, nothing was more important than protecting the life of the symbiont. (DS9: "Rejoined")
The idea of a Trill taboo prohibiting rekindled romance between former spouses was conceived by Michael Piller. (Cinefantastique, Vol. 28, No. 4/5, p. 44) He suggested it early in the run of Star Trek: Deep Space Nine. René Echevarria later recalled, "He felt they'd have a very strict taboo, in order to avoid an aristocracy of the joined. Otherwise, they'd only want to hang out with each other, their dear old friends from five hundred years ago and it would become a really screwed up society." (Star Trek: Deep Space Nine Companion (p. 279)) The DS9 writing staff in general accepted the notion that renewed attraction between former spouses was forbidden in Trill society. "We always suspected that was a Trill cultural taboo," reflected Robert Hewitt Wolfe. (Cinefantastique, Vol. 28, No. 4/5, p. 44)
From as early as when the story was originally concocted by René Echevarria, the idea of Trill taboos preventing Jadzia Dax becoming romantically involved with another Trill was part of the plot that developed into "Rejoined". (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 107) "The existence of the taboo meant that we could do a tragic love story," Echevarria acknowledged. (Star Trek: Deep Space Nine Companion (p. 279)) Robert Wolfe wondered, "How do you make the audience understand that even though Dax is doing something they would find acceptable, it's against the norms of social behavior for her society?" The writing staff's solution was to make the relationship a homosexual one. "[The taboo] was always an analogy to the way homosexual relationships are treated in our society," Wolfe explained. The inclusion of the banning of reassociation in the story also served to highlight the alien nature of the Trill. (Cinefantastique, Vol. 28, No. 4/5, p. 44)
Immediately, Ronald D. Moore liked the concept of the taboo, later saying, "It's a great metaphor for today, but it wasn't hitting it over the head [....] You had this taboo, which sounded so ridiculous, on Trill, yet it makes you think our own taboos are ridiculous. I thought that was a great way to go." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, pp. 107-108) Moore also regarded the taboo as "a perfect metaphor for issues of sexual tolerance and intolerance." Expressing his approval while "Rejoined" was in development, Moore told the other DS9 staff writers, "This taboo tracks with our own taboos or many audience members' taboos about homosexuality and the argument will track straight down the line and it will be great." Thus, Moore and René Echevarria, while writing the script for the installment, "played that [Trill] taboo without really playing our taboo" of homosexuality, even though the other Trill Jadzia Dax was reassociating with, in the episode, was female too. (Cinefantastique, Vol. 28, No. 4/5, pp. 45 & 44)
Showrunner and Executive Producer Ira Steven Behr also approved of how Trill conventions were used to metaphorically represent homophobia. "It was a Trill problem [which is established in 'Rejoined']. It wasn't about sex. It wasn't about same sex relationships. It was a distinct, science fiction, Trill problem. That was good, to couch it in futuristic terms," he remarked. From the perspective of her character, Jadzia Dax actress Terry Farrell likewise clarified, "It really had more to do with my vows, being a joined Trill." (Star Trek: Deep Space Nine - The Official Poster Magazine, No. 5) She felt it was extremely "important" that the Trill taboo, rather than homosexuality, was the source of conflict in the episode. ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features)
Some things about reassociation seem to conflict with what was seen in TNG: "The Host", when the Odan symbiont appeared willing to rejoin with Beverly Crusher after its transference into a new host (though the way Trills were portrayed in this episode contradicts almost everything subsequently shown about Trills). This law does not seem to apply to platonic friendships: three of Dax's hosts were friends with Benjamin Sisko and Jadzia did not hesitate to rekindle Curzon's friendship with Kang, Kor, and Koloth. Ezri Dax's decision to remain with Jadzia's old friends appeared to be perfectly acceptable to Trill society, apart from some initial discomfort among the DS9 crew about how to relate to someone who both was and wasn't their old friend. This is also consistent with Michael Piller's original notion of the taboo, intended "to avoid an aristocracy of the joined." (Star Trek: Deep Space Nine Companion (p. 279)) Relationships, especially platonic ones, between a joined Trill and a non-Trill (or even an unjoined Trill) would not present this risk. It seems also acceptable for joined Trills to be platonic friends with the spouses of their former hosts. When Worf wanted to avoid Ezri, Captain Sisko assumed it was to honor the Trill custom forbidding reassociation, Ezri corrected him by saying "that doesn't mean we can't talk to each other." It is possible that this law only applies to two joined Trills, as Ezri's decision to have sex with Worf did not weigh as heavily on her as Jadzia's feelings for Lenara did. When Worf stated that she had broken Trill law, she neither confirmed nor corrected him, she only responded by saying that he had "broken a few Klingon laws in his time." All of this suggests that a joined Trill reassociating with a spouse who is not also a joined Trill is not acceptable but not as strictly forbidden either (though if the purpose of the custom is to prevent an "aristocracy of the joined" it would not further this purpose to hold an accidental host, like Ezri, to the same standards as a host who was properly screened and chosen, Ezri may have had a reasonable expectation of receiving leniency for her transgression).
In the novel Triangle: Imzadi II, Worf speculated that his marriage was over, as the new Dax symbiont – even assuming that it was a female – would be automatically discouraged from a further relationship with him, due to the rule against reassociation. This could be the result of his ignorance of the fine details of Trill society, where this rule is concerned, rather than definite proof that romantic relationships between Trills and non-Trills could not continue past the life of one host.