Why do dogs prefer other dogs food to their own - even when its exactly the same?

I've seen this happen so many times and so very often in my own family. I mean, my son does it all the time. He's always looking after what his sister gets because he thinks she always gets a better deal. On the other hand my daughter doesn't even care about it. It's awkward, but dogs seem to have the same attitudes towards their fellow dogs.

If you put two dogs in a same room and feed them the same exact amount of anything they will –without a doubt- end up trying to eat their neighbor's food.