Military: War
Despite their predisposition for emerald forests and lofty mountains, many elven kingdoms were interested in what the unknowable depths of the Underdark had to offer and made many excursions beneath the surface, specifically the vast deposits of the magically attuned substance, mithral, which the dwarves jealously guarded.
Centuries of encroachment on dwarven territory eventually boiled over into a full-blown war between elves and dwarves. And thus, a timeless stereotype was born, as the elves and particularly the dwarves have held onto a grudge ever since.