You are expecting him to put you on a worse pair than he has, or an overs and a flush draw sort of hand.
There is a slight possibility that he could put you on 88, and an even slimmer possibility that he puts you on 67 suited...but I think he calls here almost every time if he has AA, and most of the time if he has KK.
Good play. If he didn't call, which I doubt, then he had missed overs or he is super ultra conservative.