On the top of my mind I can’t really remember any particular important demotivational experience. That may sound strange but depending on the kind of environment that one grew up experiences may vary. After attending schools in a small and isolated town where many teachers had a second job and were there just to pass the day, I learned very quickly not to expect anything from them.
As you can imagine, it’s very hard to get disappointed if you don’t expect anything at all. I got in the habit that if I wanted to learn something, I had to do it by myself. And so no, I don’t have any story about this or that college professor that made me loose interest in his subject. When I entered a classroom I was either self-motivated because I knew that their particular subject was important or interesting to me, or I wasn’t at all.
What I can remember very well though are the professors that motivated me to learn something that I mistakenly classified as non-interesting. The one thing they all had in common is that they introduced their subject with real-world applications; they made me realise what sort of non-synthetic problems I could tackle after taking their class.