This is too long, I know. After some complaints that people were having trouble with the archive.org embedding I decided to try youtube. Hope it works better now.
My de-motivational experience was during my high-school course on linear algebra. It was a painful course where we were expected to learn a series of arbitrary rules and solve endless problems using a particular method — I think Gaussian elimination. This was extremely tedious and when I asked what matrices could be used for, the teacher could only tell me that they were ‘good for engineers’. As a result, I didn’t take any further mathematics until I went to university.
I think the situation would have been a lot better if the teacher had provided more background, both on practical applications and higher abstractions. Years later, after taking abstract algebra, I returned to linear algebra with the idea in mind that it was just a particular subclass of something else that I already understood, and it really made a lot more sense to me.