It's rude to destroy something someone has made.
During his weekly art class Ross observes that the painting Carrie is making is much better than his.
Ross wants to have the best painting of the night.
Ross asks Carrie for advice with his technique and is able to make his painting superior to all of the others in the class.
Ross stands up while holding a jar of paint and intentionally spills it all over Carrie's canvas while making it appear to be an accident.
Ross is very proud and Carrie feels flattered that he went to her for advice.
Ross's painting is picked as the best of the evening, but Carrie seems very distressed and disheartened.