"Just say I looked pretty well and was having a good time.""I'll say the first with all my heart, but how about the other? You don't look as if you were having a good time. Are you?' And Laurie looked at her with an expression which made her answer in a whisper...
It is important, that it is Laurie who addresses Meg's behaviour, because Laurie is familiar with 'vanity fair'. And, because he is familiar with it, he knows of it's phoniness, and because he knows Meg, he has to tell her, that it is not her true nature, to be spoiled and superficial. In this very moment, Laurie was the right choice to remind Meg of who she truly is.