Most re-homed dogs have what we call a "honeymoon period." This period varies from dog-to-dog, but generally lasts a couple of weeks to a month. Shy dogs will be in the honeymoon period longer, and gregarious dogs less. The honeymoon period is a time when the dog isn't sure of his status and is on good behavior.  

This is the time your new dog is learning the house rules, family routine and simply appreciating their new found security. This is why it is essential to set the rules from the start.  You don't want your new dog on the furniture, then from the beginning don't let him up on the furniture. You want him to eat dinner at 5:00 then start on day 1 feeding him at 5:00.

This is also the time to start training and setting up the environment to discourage unwanted behavior.  Refer to the Training section for more.