If the visitor says nothing and eats politely, something is wrong. That realisation sends the world of the true Auntie crashing down about her, and the sun retreats behind thick dark clouds of disappointment. But if the visitor's face goes all dreamy and his/her eyes close slightly in order to better savour the activity that's going on in the mouth... why... then a song begins to hum in Auntie's head. And she carries that little ball of happiness all over the house for a day.
After that, the Auntie needs to find another hapless victim in order to recapture that same thrill. It's all very impermanent, and one must keep hunting down and cornering the unsuspecting friend or acquaintance or soon-to-be acquaintance in order to feel alive again. The best of all victims are the teenagers on the brink of adulthood. Young, innocent and unsuspecting half adults who love food with all the naivete of a metabolism that burns up every calorie fed to them.
The Auntie strikes because she knows that the calories will pile on her, and so she chooses to enjoy what others eat. Makes sense?