> But what kind of sympathy can regular people give without falling into the logic that everyone is entitled to a partner?
I think it is a leap to assume sympathy would escalate to entitlement. As deeply social species, it is in everyone to need love, both receiving and giving it, to have intimacy. Entitlement is a value judgment that should not be relevant to have empathy for this; when people's deep needs are unmet permanently, they hurt, and in turn that hurt can turn back to society. (Not talking about anything extremist just to be clear, I mean it clearly shows in our mundane daily lives if the people we interact are healthy, content, loved people vs not.)
I agree that having a romantic partner is often touted as an absolute normativity, and people often make the mistake that a romantic partner can fulfill all of their affiliative needs, but I think it is an artifact of our hyper-individualistic settings where there is rarely an alternative, rather than mere entitlement.
And assuming therapists can contain and heal this "hurt-payload" of the entire society is naive IMHO. There is a better, peer-to-peer topology that scales and to me it is best expressed in words "Be kind to one another, tenderhearted, forgiving one another" (I am not even Christian and not trying to invoke religion here, just really like how this particular expression captures the idea). This agapic love (as opposed to consumptive, erotic love) might be the cure. To answer your question, I think the best we can do is "We are sorry you are hurting in loneliness, while we hope you find love in whatever form you need, don't forget we also love you in our own, fellow human way, why don't you come over for board games and dinner because we enjoy your company?"