Why are dogs so frisky?

When was the first contact between men and dogs?
And why dogs, even the most ferocious ones, are playful and childlike with their owners and close people?

Konrad Lorenz, a famous Austrian ethnologist from mid 1900's, winner of Nobel prize in Physiology and Medicine, has a theory for this fact.
In his book "So Kam der Mensch auf den Hund" ("And so humans met the dog") he suggests the animals, even the adults and more ferocious, became more docile because there was a genetic selection artificially caused by mankind. Approximately 15,000 years ago, when men used to hunt, jackals (wild dogs) used to follow those hunters to feed on leftovers. More than that, jackals also helped men stay safe with their barking as a signal of warning.
Humans started getting close to those dogs and the first puppies were then taken home, domesticated and the mating between males and females underwent some type of control. 

There were so many genetic alterations caused by natural selection that a "speciation" was resulted, i.e., there was the emergence of another species of dog which is today the "canis familiares" (the species with all is different breeds, approximately 400 nowadays).

No matter what his age is, the dog presents a playful behavior typical of pups because the childlike characteristics have been preserved up to adult age. Since he is a sociable animal, he has adopted men as his leader.
It is so much observed in various behaviors that we tend to consider expressions of joy and playfulness in dogs when in fact they are submissive behaviors in relation to the leader of the pack.
For example:

1) Why do dogs wag their tails?

Wagging its tail, the dog spreads its phen-hormone (typical smell of the animal secreted by a gland in the base of its tail), and that is an expression he is asking to be acknowledged as a member of the pack.

2) Why do dogs lie on their back (belly up) and stretch their necks backwards?

 This is a posture of inhibition of the leader's aggressiveness in relation to the submissive. It happens in nature to prevent fights from happening among dogs in a pack because the subordinate dog is exposing his most vulnerable part to the aggressor.

.