Wow, that was a very good and unexpected episode from a show which has only coasted along thus far. On a show that's billed as a comedy, its best episode is also the most serious. "The Weekend" is a sad episode--shockingly sad the more you think about it--and yet it never gets into depressing territory. Amy's eternal optimism shines through even when her words at expressing the exact opposite.
We intimately see and hear Amy regarding Levi, and she really cares about her. It may be nostalgia, but it doesn't make her feelings any less valid. And Levi, as we see, is perhaps an unwinnable case, which he states himself. Amy, however, remembers those times with him and continues to believe he can be better.