Rivalry. It is an interesting concept. Some are forged due to minimal distances between two clubs. Some are created due to the politics or economics within the game.
Arsenal-Tottenham, two clubs sharing one region of London only 15 minutes apart. It all started when Arsenal had to relocate north of the River Thames over 100 years ago thanks to financial difficulties.
Liverpool and Manchester have a feud as two neighbouring English cities due to economics. Liverpool, a historic port city. Manchester, home to textiles and cotton. Manchester created a canal that meant ships could bypass Liverpool, carry on up the River Mersey and directly arrive in Manchester.