Because on my keyboard (french keyboard, win7), when I press repeatedly on Shift, Windows display me some shitty stuff about "remanent keys" or whatsoever. I'm sure I can fix it on my windows, but it would be easier if I could change the controls of the game.
Oh crap, I totally forgot about the "sticky keys" on Windows. I wouldn't manage to do full key customization so I've remapped second jumper's control key to SPACE. I hope it won't activate "sticky keys" for you.
And I'm glad that you like my game.