I get the sense that Andy is one of those who believes that a person can only truly love (in the romantic sense) one person at a time. When he is in love with someone else, he cannot "still" love Brittany.

Maybe he's trying to seduce someone. Sure, he "still" very much loves Brittany, but look! He forgot about her anniversary! He calls her a friend, albeit a "still much loved" one! There's hope he might love again!