The dishes are the dispute
Dee brings everyone to therapy after a dinner collapses over who must wash up.
Supporting source ↗CASE FILE · S08 E05

The Gang gets Freudian as they step into the therapist's office to work through their mountain of issues. But when Dennis thinks he has everyone figured out, the shrink turns the tables on him and exposes the insanity in his unique brand of pop psychology. Dennis' psychology is exposed as abnormal indeed.
THE SHORT VERSION · SPOILERS AHEAD
Dee brings everyone to therapy after a dinner collapses over who must wash up.
Supporting source ↗Mac wants size; Charlie takes acceptance as permission to intensify his habits.
Supporting source ↗Dennis brings psychological files on his friends. The therapist instead questions his secret dosing of Mac.
Supporting source ↗The therapist assigns Dee the dishes. Dee responds by smashing them in the office.
Supporting source ↗THE EVIDENCE
Charlie sits alone on the couch during his session.Source ↗
Dee begins by challenging her therapist.Source ↗
The therapist listens to Mac from her armchair.Source ↗
Dennis takes notes during his appointment.Source ↗
Frank lies across Charlie’s lap in the waiting room.Source ↗
The consulting room is crowded by all five gang members.Source ↗
The therapist holds one of the files Dennis brought.Source ↗
The therapist questions Dennis about secretly dosing Mac.Source ↗
Charlie describes the unpleasant jobs assigned to him.Source ↗
Mac laments the loss of his intimidating size.Source ↗
Broken dishes cover the office floor after Dee’s outburst.Source ↗
Dee borrows a scene from the film.Source ↗
Tags document researched occurrences, not every appearance. Read the research notes and attribution.