Your headcanons

Grumble Volcano is how I imagine the planet were Mario and friends lives on looked like about 252 million years ago.
 
Bowser's not actually evil. He's just really bored and creative. So he comes up with all the schemes to alleviate his boredom and get an adrenaline rush. He's also much more powerful than Mario, so he gives Mario obvious ways to win, and even gives hints, and Bowser has his subjects do the same.
They all have infinite 1-ups, so they don't have to worry about casualties. The only dangers are when the fabric of the universe get put in danger, such as in Super Mario Galaxy.

Peach and the Mario Bros. put up with his antics because they also rather enjoy the adrenaline rush of adventure. Without their adventures, they'd be living the dull lives of dealing with politics and plumbing work, respectively.

Bowser just puts on a show of being a bad guy in order to give the media something to cover.
 
Magikrazy said:
"Mario! The Prince of Nigeria called and he said he just won the Spanish National Lottery and he's gonna split the money with me! He promised me it was legit this time!"
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Well the only thing Luigi told him was "I just won a huge mansion, meet me there and we'll celebrate."

He never said who gave it to him nor that he won it in a contest he didn't enter.
 
I have a bunch of headcanons related to music from real life for some reason.

-Like I said earlier, Mario's favorite song is Stayin' Alive.
-Mario does want to propose to Peach, and when he does, he wants to play Together Forever while he talks.
-Bowser really enjoys listening to music when he has nothing else to do.
-Luigi hates the song He's So Shy because of people always asking him if it was his theme song.
-Wario's favorite song is Money by (I think) Pink Floyd.

And then some birthday headcanons I have.

-Mario's birthday is July 31. (the day Donkey Kong was first released in US)
-Luigi's birthday is July 20. (the day Mario Bros. was first released in US)
-Peach's birthday is November 15. (the day NSMBW was first released in US)
-Bowser doesn't have a birthday. (he's always existed, and always will)
-Yoshi's birthday is March 10. (Yoshi's Story in US)

And finally, a couple of plot headcanons.

-Bowser only kidnaps Peach at her will because sometimes she wants to get away from Mario. She looks so desperate in games because she doesn't want to go back to him.
-Mario doesn't actually enjoy parties.
 
Rick Lax said:
-Mario's birthday is July 31. (the day Donkey Kong was first released in US)
-Luigi's birthday is July 20. (the day Mario Bros. was first released in US)
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Mario's birthday, according to Nintendo Power Nsiders, is October 11th.

And if Luigi is Mario's twin, it's also October 11th.
 
Huh, never would have guessed October 11. My headcanon for birthdays was also somewhere in July (although a bit earlier, like the 10th or 12th, I forget).
 
My headcanon would've been March 10 because even Nintendo thinks that's Mario Day now.