Reassociation

From Memory Alpha, the free Star Trek reference.

Reassociation is the resumption of a romantic relationship between a person and at least one joined Trill by its subsequent host. Platonic friendships without a romantic element are not subject to this rule, and may be resumed. Since the point of each successive host is for the symbiont to acquire new lifetimes of experiences, Trill consider re-connecting with relationships from a past life "unnatural". Joined Trill who reassociate are exiled from Trill society, and their symbionts die with the current host since they no longer have access to new hosts. This is a dire consequence, since for Trill nothing is more important than protecting the life of the symbiont. (DS9: "Rejoined")

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. It is possible that this law does not apply to non-Trill.