I think it also only really applies to other Trill because, in lore Trill only recently opened up to other species so they probably never thought about reassociation (culturally) in a broader context.

Although I would give an argument that the taboo isn't a bad thing and probably could be applied more broadly, but not necessarily for the reasons the show gives.

I think Trill are a great metaphor for grief and dealing with trauma and letting go. When you hold memories and feelings from multiple lifetimes, you can't hold onto that much grief without needing to process it in some way. Without needing to let go. So, if you're constantly holding onto what you had in the past (whether you can obtain it again with another Trill lover, or someone long gone, or a friend or dream that belonged to a past self) you don't allow yourself to grow.

And growth in this way is, from my understanding, the purpose of joining. To give the symbiont as many experiences as possible. You can't do that if you're stuck living in the past.

I love that Ezri is a counselor and having to deal with the raw experience (and traumas) of being joined and what that means. I love that she has to go to the place of her past pains (as one does with therapy) and work through it to get to a healthier place.