That's what I hate/ love about SnK, it shows that the hero can't save everything and anyone(even though it sucks). The hero is faced with the most tough decisions that he doesn't want to make. But it makes this whole show/manga so much more life like and amazing.
I find it weird to ship Levi with anyone (Not even Ereri) like, it feels like he has lost too many people to even want to love others so titans' the perfect match for him lolol. But Levi x Petra doesn't sound half bad ;)