Maintaining a good instructor-student relationship ideally means establishing mutual respect while being personable. However, to be professional this relationship must be in respect to the classroom only. To extend the relationship further would invite conflicts of interest, decline in other student morale, and would undermine your obligation to being an unbiased authority.
Maintaining a professional distance ensures that you can morally critique the students performance without bias. Furthermore, it ensures that you maintain a professional image with the students.
