Forum:Snape and Harry

There's been something I've been trying to get my head around ever since I first read Deathly Hallows. When Dumbledore asks Snape if he has grown to care for Harry, Snape shows him his patronus in answer (doesn't he also say "always?"). I know this is Lily's patronus, but how does this relate to how Snape feels about Harry? It's not that I'm questioning whether Snape cared for Harry, because I believe that he did, I just don't get how Snape is using his patronus to answer Dumbledore's question about his feelings for Harry, since the doe could just be about Lily. Thanks!