It wasn't whether it was a happy ending or a sad ending in the traditional sense of story telling.
In the last of us, its really about the relationship between Joel and Ellie. The infected, the fireflies, and the world they live in aren't the primary focus, they are things that help move the plot forward.
My dissatisfaction is that ending was ambiguous. You don't know if Ellie accepted Joel's statement as fact or whether she knows he was lying and still went along with it.
Their relationship evolved where they became truthful to each other. Joel opening up about his daughter and Ellie revealing the story about Riley. So when Joel lied in the end it felt like he was a different character because it went against how he progressed as a character.