Why does Brock leave Ash?

Why does Brock leave Ash? Near the end of the Sinnoh saga, Brock began to show interest in becoming a Pokémon Doctor, and so, rather than following Ash to Unova to advise him, decided to remain in Kanto to study.

Near the end of the Sinnoh saga, Brock began to show interest in becoming a Pokémon Doctor, and so, rather than following Ash to Unova to advise him, decided to remain in Kanto to study.
See also  Why does the UK get Marvel films first?