Dogs in dreams usually speak about happy situations in your life. The dog symbolizes most of the times your friend or group of friends. By analyzing the dog's attitude in your dream, you can 'read' your friends - if the dog is playing or happily barking it means that your are surrounded by friends who love you and wish you the best all the time; if the dog is big and powerful, imposing and has a friendly attitude towards your person this means that you have a very influent protector who is going to help you when in need.
On the other hand, if the dog in your dream is snarling at you, chasing you or wants to bite you, this should be interpreted as a forewarn about your friends, as not all of them are sincere and speak their minds, they may even plan treachery. A dead dog in dreams signifies the loss (not necessarily death) of a dear friend, a conflict you have with a very good friend etc. If you buy or attempt to buy a dog as a pet, it means that you want to gain friends by doing them favors, including by offering them large sums of money.
The dog in your dream may also symbolize the social aspect in your life and how sociable you yourself are - and here we have the same interpretation of different situations: if the dog is playing and happily barking, then you are very sociable and have many friends; if the dog is vicious, chasing or snarling at you or another individual then you are not a 'people's person' and you should try to be more friendly and communicate more with the others.
The image of a black dog is very common in our dreams - it stands for negative feelings and a negative attitude you adopt towards life, including anxiety, depression, pessimism, grief, lack of confidence in your abilities etc. The black dog mainly signifies a metaphoric death, like the 'death' of feelings, passions, trust etc. Nevertheless, there are cases in which a black dog, especially when the animal is hurt, ill, dying, forewarns about the future death or serious health disorders of someone you know.
Dogs on leashes appearing in dreams symbolize the instinctual human nature you are trying to cover up and suppress by resorting to reason and intellect. If you carry a dog on leash and walk peacefully along an alley in the park, this means that you are going to successfully repress your primitive desires and control the 'beast' inside. But if the dog in the dream is fighting to release itself and get out of the leash, then you should be more careful about those untamed, 'animal' desires. Also, carrying a dog/ puppy in your arms means that you succeed in repressing your instinctual side.
Being bitten by the leg by a dog forewarns you about the fact that your instincts and primitive desires may break the 'surface' of reason and cause you many troubles. However, if the dog bites you by the hand this is a stronger warning that your friends may soon turn out to be your enemies who have treacherously been planning to cause you many problems.