I'm not sure that it's any kind of smugness. Rather, I think it's because he knows that he's engaging in something that the goo-goos would consider "negative campaigning," and he's worried about coming across as unlikable. John McCain no doubt knows that he's had a slight nastiness problem in the past -- remember his final Republican debate at the Reagan Library? (Side note: Perhaps he shouldn't worry . . . it seems to rear its ugly head more when he's dealing with members of the GOP than with Democrats.)
In any case, the inappropriate smiling is a tic McCain is going to have to watch. The MSM is already prepared to label him as an old man. The last thing he needs is for that epithet to be preceded by "weird."