By the "natural notes" I just mean the notes that would make up an octave in that key, like "Do, Re, Mi, Fa, Sol, La, Ti, Do" in whatever key you like. On a pia

By the "natural notes" I just mean the notes that would make up an octave in that key, like "Do, Re, Mi, Fa, Sol, La, Ti, Do" in whatever key you like. On a piano, the long (white) keys are tuned to C for that. If you start on a D and just play the white notes up, it won't sound right. If you're asking "why is it tuned to C", it's probably because C is the key that historically was assigned the letters without any sharps or flats. There's no physical reason for this, it's just a convention (as opposed to, say, octaves, where each octave is half the frequency of the previous). If you're asking why C got the letters without sharps or flats, I believe you have the Romans to blame for that. They did start with A for their scale without sharps or flats, but it was what we would now call "A minor". Which leaves C as the major scale.