Quoted By:
She always wanted to when she got a Mawile for her 8th birthday she is currently 15 from her rich folk parents but she never really had a DIRECTION. She didn't want to take the plain jane Pokemon League route, she hated how it felt like it was to outright baby and coddle weaker Trainers but she lacked a direction to take her first major step towards. But being a girl raised in Castelia City she eventually had a run-in with Team Plasma and decided that after running them out of town she'd let the first major leg of her Pokemon Adventure be a campaign against Team Plasma. Otherwise all she cared about was getting stronger, improving and staking her claim without having her hand held by the Pokemon League. Taking down Team Plasma since this follows more of a Pokemon Generations plot line as far as events in Unova are concerned, which left the spot to spearhead going against Team Plasma rather open was simply self-affirmation that she was genuinely as powerful as she thought she was.
From there the motive was simply that actual PHYSICAL power means absolute freedom and never having to live under the thumb of someone else in her mind. So she wants to keep getting stronger so she can always be free to do as she pleases and go on any sort of adventure she wants or can come across, living to her own whims based on what pleases or displeases her.