For Mario, I totally agree. But Bowser is a king himself, so why would he fall in love with a princess just because she's a princess? I think his plans to marry her are rather for geopolitical reasons: A marriage would lead to a merge of both kingdoms, and the first born Bowser Jr. (who doesn't have Peach in his blood line) would eventually inherit this bigger kingdom.
Mario needs to save himself from the pointless questing at endless castles. Bowser needs to save himself from his own tacked-on pomp & patriarchy so he can love the tiny koopa inside his overlarge shell. The both of them have some work to do. Meanwhile, Luigi & Toad could use a fuckin' buddy movie. Just sayin'.