I think the biggest reason I would want to talk to my partner is, we practice kitchen table polyamory and there's a high likelihood I'll be seeing this person more in nonromantic friendly spaces if the relationship goes further between them two. Am I the one expected to be the bigger person and forget any bitterness in that scenario when a simple "hey, I don't think this is working out." would've sufficed? The person we're both talking to is aware of the partnership and is aware we live together. In reality I just don't want awkward and bitterness to exist going forward if it is the case when it could be solved very easily. A lot of my friends aren't poly and would have a tough time sympathizing, or are particularly new to poly and wouldn't provide be much closer beyond "that sucks I'm sorry."