Both Apple and Google (and Samsung most of all) realize that making a watch that only works on their platform helps to lock you in to that platform, and helps them with future sales. What motivation does Apple have to make the Apple Watch work on Android (or Windows or BlackBerry). Answer NONE! Yea in the short run they might get a few more watch sales but in the long run they are much more motivated to keep you buying Apple. Or Samsung, or Android, etc.
Fortunately we have 3rd party companies making watches also such as Pebble and Martian, so that those of us who are platform agnostic can gave our cake and eat it too, when we constantly change phones.