A phrase originally meaning whenever
someone can't stop obsessively thinking about something or
someone, particularly something unimportant, or
someone who isn't even remotely thinking about them in return.
However, due to overuse by morons, has come to just be a regurgitated, automated response to criticism of something someone doesn't want criticized. Often used as a coping mechanism whenever an actual, intelligent rebuttal can't be made.
Person 1: "I
don't like this restaurant. Every
time I go there, the tables and chairs are
sticky and the service gets my order wrong."
Person 2 (smugly believing they are somehow making a point): "Oh yeah? Well, that restaurant is living in your head RENT FREE!!!"