Why does my dog vomit every time he gets a very slightly different bag of do food?

...And why are these animals so cute?

http://media.use.com/images/s_2/ed15...ec30d9bf_1.jpghttp://media.use.com/images/s_2/ed15...c30d9bf_12.jpghttp://media.use.com/images/s_2/ed15...c30d9bf_16.jpghttp://media.use.com/images/s_2/ed15...c30d9bf_18.jpghttp://media.use.com/images/s_2/ed15...c30d9bf_27.jpghttp://media.use.com/images/s_2/ed15...c30d9bf_36.jpghttp://media.use.com/images/s_2/ed15...c30d9bf_37.jpg

When changing foods it is always recommended you feed a tiny bit mixed with the old food and slowly increase it over a couple of weeks until the dog is on the new food.