[Eric18]

My experiences match Dave's. Perhaps the university setting requires a more distant relationship due to numbers of students and procedures, but teachers have to find a way to both build rapport and maintain some distance. The power dynamic also complicates matters because you are giving grades. Finally, some students will seek recommendations and/or waivers.
When I taught at adult or IEP programs, you could create a more casual atmosphere. But, again, professionalism dictates a certain distance.
Having said all that, I consider several former students friends and have maintained contact with a wider circle through occassional emails.