Princess Peach Toadstool was never a fighter. She wasn't weak or a pushover, but she was still a princess. Royalty was too important to risk their lives fighting, that's what the retainers were for. And since she'd never been taught to defend herself she was a prime target for kidnapping.
Bowser was bad enough, but the last straw was Booster. Idiotic, creepy, possessive, frightening Booster, who wanted to marry her just because she'd fallen from the sky and into his tower. She hadn't even tried to fight back, and that was the worst part. All she'd done was cry and wait for Mario to come rescue her, and after he did, she made a vow to herself: from now on, she would fight back.
Even if she wasn't as good with combat as Mario or his friends, she could learn. She didn't care if it wasn't proper, if a princess wasn't allowed to fight, she was going to help save the world this time instead of watching on the sidelines.
"Thank you, Grandma," she said as she prepared to leave. "Thanks for putting up with my selfish needs."
Her grandmother only smiled.
"It's no trouble, dear. See the world, make new friends...defeat this Smithy."
When the Mushroom Kingdom next saw its princess, she was a hero.