Why do you get those sudden shocks when you realize you knew sorrowful things?

I was listening to one of those vocaloid songs (http://www.youtube.com/watch?v=JCkByKskziw&feature... which for a while I was liking, its a beautiful story, though it is sad the first time i liscenend to it i shed a tear.

Then, I went to my history class (High school) and learned the place, and that it was tied to something else i knew, though it was just has unclear. The song was about a prisonier in a nazi execution camp (1949, World War II, the genocide of the Jewish)

Since then, Nazis are infinitely horrible, and this song makes me cry. everytime.

Why does learning 2 elements and then learning their bonds makes things worst/better so suddently?