The thing I loved best was the music. Scenes 1, 3 and 6 I especially liked because the music complements the action without sound effects (also, you have an awesome array of modern effects that fit perfectly with the piano melody). Honestly, this is what every Flash artist should be able to do, create their own music to fit perfectly with the scenes.
Another thing, you created a fantasy effect by having a shape-shifting dog. Coupled with the music and the girl having fun, it gave off a nice effect. It also seems to have a purpose, that the "smart" side (two legs, like a human) would guide the girl, and the "wild" side (four legs, like a beast) acts as a playmate.
The only part I did not like was scene 5, when you have a flashback like an old movie reel, and then you destroyed the music-only effect you have in the other 5 scenes.
The last scene is the type of cliffhanger that makes us beg for the next movie. I hope I'm not reduced to that.
OH GOD, PLEASE MAKE THE SEQUEL!!!
As a final note, I cannot stress how awesome the music was, and that you made it yourself. How did you come up with the melodies and what instruments to include, and make it work so incredibly well?