What happen when you leave your motorcycle sitting so long without starting day by day?

That's correct for a few days or even weeks but leave it months or years and a few things happen depending on where it is kept. Over a number of months the battery will slowly go flat. Also any fuel left in the carb(s) will evaporate and can leave a residue that will block the jets when you try to start it, which will either make it not start or run rough. Leave it for years and you will find corrosion can stick the rings and piston in the bore and it wont turn over any more. There is plenty of information about on how to "Lay up" any vehicle that will not be used for an extended period.