I am not fortunate enough to be able to have a dog at home but enjoy looking on doggie sites and seeing doggie/human tv - eg dog borstal and dog whisperer etc. However, I feel that although a lot of a dog's behaviour in the home or whilst out with its owner can be attributed to the human in charge, there is something in what cassie says too. Dogs do have personalities, it seems to me. There are some dogs who will be every so quick to learn and want to please their human whereas others are just a bit clueless and dull or perhaps they are just downright naughty and need more time/hard work.
