Whether it's puppies, kittens or children, Nature insists on play. Play is a requirement for healthy development of a loving personality as well as for a healthy body. In addition, play is the basis for a social structure. Individuals of any species that do not play when they are young are severely mentally and socially compromised when they become adults. Socializing should begin no later than three weeks in kittens, or six weeks in puppies.

Actively encourage your dog to engage in appropriate play from an early age. Provide him two or three safe toys to play with at any one time. Praise any interest or chewing on toys.

Pets who learn to play early, and play a lot, tend to retain that playfulness later in life. This has several benefits to the pet:

• Cardiovascular development
• Balance and coordination
• Joint lubrication
• Hunting and fighting skills
• Easier and more fun for the owner to exercise the pet (e.g., throw a ball, or drag a string).
• More fun in life for the dog
• Owner usually enjoys watching (and participating), so pet is more entertaining. This increases the owner-animal bond.

Though a related species, dogs play more than wolves. In wolves, play decreases soon after puberty. In dogs, we have bred/selected over time for playfulness to continue throughout the animal's life. This is no accident. Humans have bred dogs for this "juvenile" behavior, because we value playfulness as a personality characteristic in pets. We should provide toys and encourage non-destructive play as much as possible, starting as early in life as possible. In genetics, when a juvenile trait is selectively bred, the resulting species is said to be "neotenized." One animal behaviorist jokingly wondered aloud, "Are humans really neotenized chimpanzees?"

Aggressive play allows the dog to show aggressive symptoms toward humans, and enjoy doing it!   This may contribute to dog aggression in the household setting.

Aggressive Play
In cats, because they are not pack animals, kitten play in the wild gradually becomes more aggressive to drive away potential hunting competitors. This results in distinct hunting domains, and is the reason cats are often referred to as "asocial." The reason many cats are so social with people is that they perceive them as "part of the family" and therefore fully accepted and loved. This can be undone by aggressive "slap" boxing by people playing with cats.

Play fighting between puppies usually does not result in an aggressive adult dog. It's a way for puppies to determine their rank in the litter and for adult dogs to cement theirs with the pack. However, rough play between people and dogs over 6 months often turns out badly. The aggressive threshold is lowered because the dog is rewarded by the fun of growling and tooth grabbing a human. Guard dog breeds have a genetically lower aggressive threshold, so they love the rough play, but are also more likely to escalate the aggression later, but usually toward a different human.