1.they do this because they believe you are more important than
them and, in a wolf pack, the more important wolves get better
food. so even if you have the same dog food in your hand as in the
bowl, they believe it tastes better. then when/if they get it, the
placebo effect makes it taste better
2.they know you are more important than them and feel somewhat
honored by your gift of your food
3. your food tastes better than what is in their bowl. like
chicken tikka musala is better than raw potato
4.your hands have touched lots of interesting things, so the dog
can smell and taste the difference between the dog food in your
hand and the dog food in their bowl
5.they are plain bored of their food and want something
different for a change.
it can be one of these, a few of these or all