From Bulbapedia, the community-driven Pokémon encyclopedia.
From Generation III onwards, a Pokémon's base friendship is the value to which its friendship is set when the Pokémon is caught or received from an event or NPC. A Pokémon hatched from an Egg has its friendship set to 120, while a Pokémon caught in a Friend Ball has its friendship set to 200. A Pokémon's friendship will also be set to 70 if it is traded, regardless of the base friendship for its species. N's Pokémon start with maximum friendship.