That’s one piece, and a piece that this episode expanded upon by showing how long they had been working together/friends (over a decade).

Then there’s Bill’s note, which made the loss of Tess even heavier, but reenforced the idea that he shouldn’t be helping Ellie just because of a potential cure, but because people like them (Bill and Joel) are their to protect others, something he will feel even more compelled to do given his “failure” protecting Tess.

All of that from one beautifully crafted episode. I haven’t played the game(s), and I think they’re doing a great job of building Joel’s character and allowing the viewer to not just see him change, but also why he’s changing.