Well, I'd say this sums up their relationship.

This is probably a reference to a drama Gainax recorded on the "Addition" CD which stars the original Japanese cast of Evangelion (sans Kaji). It's called Shuukyoku no Tsudzuki (After the End), and there's a part where Ayanami starts slapping the piss out of Asuka. It's kind of pretty hilarious.

Payback from the elevator incident I suppose.