Dogs crave human contact. They'll happily roll over on their backs to get their bellies rubbed or press against your hand for a scratch on the head. But when you reach down to touch their feet, they jerk away as though they'd stepped on hot coals.
This aversion seems to be universal. Even when dogs are playing with other dogs, they usually avoid the feet. Vets aren't sure why so many dogs are foot-shy. Some may be ticklish, especially between the toes. Others many have bad memories of previous--and possibly painful pedicures.
Most dogs will put up with a little paw play if you get them used to it when they're young. Older dogs aren't so flexible, however, so your best bet may be to give attention to other parts of their bodies and leave the feet alone.