As the iconic partner of Ash in the Pokémon anime, Pikachu shares a very close bond with its trainer. Like any loyal friend, it can feel upset at times if that friendship is threatened. Some reasons the game's Pikachu may exhibit jealousy include:
When Ash lavishes too much praise on another Pokémon after a victory.
If Ash forms a strong new connection to a Pikachu rival it must battle.
When it believes its special role as Ash's original Pokémon is being replaced.
Pikachu's jealousy usually stems from feeling neglected or separated from its treasured friendship with Ash.