My demotivational story is related to an advisor. In our meetings, he would simply express concern over over the work not proceeding well enough or quickly enough, without offering details on how to improve. When I asked for advice on certain techniques, his most often answer was that I should go back to the literature. The closest he came to a vote of confidence or praise was ‘ok, keep going’. It seemed to me that he was not engaged with my project, not invested in me as a student, and didn’t care about my success (or failure).
Meetings with him were extremely demotivational. I would come out less excited about my project than I went in, and I’d spend the rest of the day (or week) trying to recover my motivation rather than working on the project. It was completely counterproductive.
Over time (and in talking with other students), I learned that my experience with him was not unusual and gained some techniques for more productive interactions with him. However, the experience really emphasized the importance of mentorship and good advisor/advisee relationships to productivity.