Joel refuses to forge connections with the potential to shatter him: despite his partnership with Tess, he kept her at arm’s length. As for Ellie? He refuses to see her as anything more than some walking-talking cargo. He doesn’t want to be attached to anyone, and certainly not a snarky little girl who reminds him of his past.
So the show makes Bill into a cautionary tale of another kind. After all, he’s been where Joel is right now: he kept himself protected, well-fed, and emotionally unattached. But in the end, none of that matches up to the life he has once he lets Frank into his bubble. Sure they bicker, unable to agree on some details of their life in the town. And yes, caring comes at a terrifying cost — suddenly, there’s something besides himself that requires protection. But that companionship and love is so much greater than a safe life of solitude. So much so that when Frank decides to end things, Bill can’t imagine going back to how things were.
“I used to hate the world and I was happy when everyone died. But I was wrong,” Bill tells Joel in his note, doing his best to impart his hard-won wisdom. “Because there was one person worth saving.” If Joel is willing to open himself up to loving someone like that then yes, it might just end in sorrow, like the calamity of Bill and Frank dying in each other’s arms. But according to Bill, that sure beats a life of bitter solitude.