Nothing to do With Reality

 Fantasy stories are fantasy stories for a reason - dragons, trolls, and gnolls do not exist in the roundworld. Fantasy is meant to be for fun, to get away from the world. However, the best stories are allegories. They relate to our world in ways deeper than a pointy wizard hats.

Fantasy is often thought of when thinking of different species. Racism is often the most common output of this trope. Elves and Dwarves being common in any fantasy trope. The biggest titles - think Tolkien - often use this difference species as a metaphor for racism, using the fact dwarves hate trolls and vice versa for their quarrels. He didn't much go into the generational hate for his story, instead focusing on the journey. However, Pratchett (you knew it was going back to him) uses his books as metaphors for religious hate, and how religion causes much more hate than we can hope to change, without fundamentally changing the structure of religion.

Spoilers(ish) for Thud!

In the story, Tak wrote the world into existence (that is why it is illegal to erase the written word in dwarvish culture). 

"The first thing Tak did, he wrote himself.

The second thing Tak did, he wrote the laws.

The third thing Tak did, he wrote the World.

The fourth thing Tak did, he wrote a cave.

The fifth thing Tak did, he wrote a geode, an egg of stone" (Thud!, Terry Pratchett).

When the geode broke open, the brothers came. One brother walked out of the cave and saw the light, and therefore became man. The second stood under the darkness of the cave, and therefore became the correct height and was a dwarf. The third, however, never made it out of the geode. This malformed creature then became troll.

This was the written word of dwarf for centuries. This was the start of hundreds of wars, and this was the stem of hatred from dwarves to trolls. 

 Dwarves condemn trolls for this. The religion had been corrupted, the fundamental parts of the religion used for malice. There are so many wonderful parts to this story - so many allegories, so many genius stories that make me tear up. Sir Terry Pratchett is one of the best authors of all time. You truly have to read his novels.