Sarcasm is an adjusted stress reaction.

I recall River saying in Forest of the Dead, "He looks right through me, and it's shouldn't kill me but it does." Putting her line in The Impossible Astronaut and together with this one, I read that as older!River having thought she'd grown past falling apart when the Doctor doesn't remember her. Coming to terms with it. And she *doesn't* fall apart. But it still hurts like hell.