When I began teaching, I had no idea if I was doing a good job or not. The first few classes were not very good. In fact, my first year, I floundered. Later I got the hang of it and felt comfortable, but did not know if I was good. This was before the age of student feedback for NAAC.
Once I got this heartwarming email from an old student on how he hoped to teach the way I did and  suddenly I felt maybe I was good. At least for a few students thought so. It also made me realise that I had lost touch with my PhD guide who had been a real mentor. So I dug out an old letter from him and renewed our correspondence, only now through phone calls. I could hear in his voice that it made him happy to hear from me. Luckily, in the last one year, he moved to Bangalore which I visit often, and I was able to meet him and his wife. I am happy I visited him a couple of times before he passed away. 
Over the years, specially on teacher's day, I get a few emails or phone calls from old students and it makes me happy. It is good to be a teacher.