Peach is the only one capable of controlling her emotions, hence why she's the heroine.
Bowser loves Peach, correct? So he wouldn't want her to be destroyed, hence why the game is so much easier: They're going easy on her because Bowser loves her.
It's not that Peach needs to find all of the lost Toads before she can enter the final level and fight Bowser, it's that she refuses to. She knows how dangerous Bowser is, and if she is defeated and/or kidnapped then all of her citizens she didn't already rescue would remain captured with no one left to save them. Can double as heartwarming if one assumes Peach acted that even if she goes down in the final battle, at least her people would be saved.
Boos that attack when you look at them instead of looking away is sheer brilliance in of itself and an idea that is very rarely seen. One cleverly designed level in Shriek Mansion shows this.
Fridge Horror: She's got a carnivorous umbrella and violent mood swings. Imagine the mayhem she could get up to if she was less inherently nice.
Also of note is that the idle animation is Perry trying to eat her. Imagine what mayhem Perry could get up to if he was being wielded by, say, Bowser.
Just how long has it been since Perry was separated from his "grandpa"? For all we know, Perry's "grandpa" has long since died of old age.
Perry talks to Peach throughout the entire game and it seems to be bonding with her ... except if you notice Peach never actually acknowledges Perry, or treats him as anything other than a parasol. It's entirely possible Perry only thinks he's communicating with Peach (like when he tries talking to the merchant in his back story), but in reality he's been talking to himself the entire time. Doubles as a Fridge Tear Jerker.