In feel fortunate that I do not recall a single significantly demotivating experience in my education (but of course this does not help now in this exercise). However I have attended many courses and seminars which demotivated me from listening or getting interested. These were courses where the teacher was or seemed demotivated. In this case it’s over. Such behavior really colors off on my motivation. I find it demotivating to see a course material which is or seems unprepared. If the teacher does not believe in the topic/approach, then it is difficult for me to believe in it. Confusing or inconsistent notation can be very demotivating. Further it is demotivating if the teacher does not adapt to the audience or does not respect the schedule/time. What demotivates me most when listening to talks is when I see arrogance towards students or colleagues.
A teacher who is motivated, who shares motivation, who is prepared, professional, and respectful can make all the difference.