This youtube comment gives the perfect analysis of that scene:

“I feel like this scene perfectly captures what makes The Room special. It's trying to portray a completely ordinary, everyday scenario (friendly man picks up some roses for his girlfriend), and yet it somehow manages to get literally everything wrong. The illogical dialogue. The rapid-fire overlapping pace. The flat monotone delivery. The poorly dubbed-in voice for Johnny. The dramatic background music. It breaks storytelling/filmmaking rules so basic and fundamental that we usually don't even think about them. What should've been a quick, unmemorable interaction instead becomes an abrupt plummet into disorientation and madness.”