I've been instructing, full- and part-time for nearly 30 years. I'm still loving it and still learning. I was trained by a man who took the view that the trainee instructor was an average pilot who would give 100% throughout the course. He was a critical debriefer because that was what we were paying our money for. He was FFF - firm, fair and, occasionally fun - and I've done my best to emulate him ever since.
As far as debriefing goes, I have always used the sandwich approach. Start and finish with praise for the good aspects (there are always some) and stick the critical bits (there are always some) in the middle. THis holds good for all areas of flying instruction.
The day you stop enjoying it, stop doing it. Happily, for most of us, that day never comes.