Princess Peach is unsurprisingly the insidious love spider at the centre of this whole web of deceit. She may maintain a regal facade, in in truth she's nothing more than a self-serving harlot with the morals of an alley cat on heat and no higher priority in life than the massaging of her own needy ego.
Let's look at the evidence shall we? She's been 'kidnapped' by Bowser a grand total of nine times so far, and that's just counting the core Mario games. Is castle security really that lame? You'd think after the first incident that the Mushroom Kingdom wouldn't be taking any chances with its monarch, but if Peach's story is to be believed (which is isn't) then the royal home is less secure than a sleep-deprived Woody Allen on a bad acid trip. No, we strongly suspect she's in on the whole sordid deal.
Getting herself intentionally kidnapped by Bowser? Whyever would she do such a thing, you might ask. Well put simply, Peach is a big old attention-whore, and the mis-guided sense of worth she gets from Bowser's actions coupled with Mario's endless willingness to come running whenever she calls gives the miserable, shallow-hearted bitch exactly the two-hit combo she needs to make herself feel good.
Why does she need the constant superficial reasurrance? Well until the Mario brothers turned up, she was the only human character in the whole kingdom. With no-one but bubble-headed fungus to relate to she must have got pretty lonely, and she now wildly latches on to any attention she gets, positive or negative, as a result. And now we come to think of it, how the hell can she be the Mushroom Princess if there's no Mushroom King or Queen above her in the royal hierachy? They may have died, got sick of her crap and left, or even sent her away to the land of 'shrooms as punishment for being a prissy wanker, but whatever the reason for their absence, a lack of parentage has probably only fuelled her needy attention-seeking.
Above: No-one knew it at the time, but Super Mario Bros' ending was a darkly prophetic metaphor
Though none of that justifies her behaviour, naturally. Her coping mechanisms have all the depth of a puddle on a hot day, and she's screwing people over left, right and centre as a result. Of course, the first kidnapping was probably genuine. Apparently in SMB1 Bowser went after her because she had the power to stop his magic. But everything since then has been for her own ends. Is it a coincidence that she actually got her shit together and helped out in the American version of SMB2, only to go into a submissive capture-frenzy once Mario went over to Sarasaland to rescue Daisy in Super Mario Land? We think not. She was jealous and she wanted his attention all for herself.
What a poor, poor sap the red-clad plumber is. Peach has got him wrapped round her pinky finger and he just can't break away. He's a good guy under the spell of a bad woman, and what's happening to him is just tragic. He and Luigi turned up in the Mushroom Kingdom with the best of intentions, and finding it in turmoil, honourably set off to help. But Peach just loved having three men (Well, two men and a giant dino-turtle) pursuing her, and thus a dark, dirty little cycle of emotional abuse was born.
It's not Mario's fault, but he's not helping himself. A self-defeating need to please is at the core of his psychological make-up, possibly stemming from depression or anxiety issues (It's a textbook symptom), and the falsely simpering "Thank You" or half-arsed peck on the cheek he gets from Peach after putting himself through Hell to save her each time is more than enough emotional pay-off for him to convince himself she's still worth it.
In his mind, it means she really does love him after all, and that any day soon they'll be together. She's just busy and has a complicated life is all. Being royal takes up a lot of her time, and he doesn't want to push her when she's getting over the trauma of another kidnapping. And if you don't think she plays up those excuses to keep him hanging on, then you're looking at this from a very naive angle indeed. It's a horrible situation, and a classic example of the abuse cycle. He can probably see what's happening, but he chooses to ignore it because he doesn't want to. As messed up as it is, there's security in the status quo, and as long as the bait keeps dangling, he'll keep taking it.