And probably also a degree of jealousy. What with Prof Oak forging his own path in research, leaving her research group do at least in part not agreeing with the angle of research, only for his 'daft idea' to help make him practically legendary in the field of Pokémon research. And then there is the fact he had the AUDACITY to retire from pokemon battling after becoming champion, a blow to her pride at not being able to surmount him, and also showing a bit of how the two see the world differently. Albeit this is mostly guesswork.It also helped that they responded to the Post War Years differently...witch makes sense.