Dogs are much more playful than wolves, and this can probably be understood as retention of juvenile behavior (called “neoteny”). Retaining existing juvenile behavior is accomplished far more easily than evolving a behavior from scratch. Many of the ways in which dogs interact with humans can be understood as a new application of behavioral adaptations designed for a pack — the owner takes on the role of the leader of the pack.

… the Russian scientist Dmitri Belyaev succeeded in developing a domesticated fox in only forth years. In each generation he selected for tameness (and only tameness); this eventually resulted in foxes that were friendly and enjoyed human contact, in strong contrast to wild foxes. This strain of tame foxes also changed in others ways: Their coat color lightened, their skulls became rounder, and some of them were born with floppy ears. It seems that some of the genes influencing behavior (tameness in this case) also affect other traits — so when Belyaev selected for tameness, he automatically got changes in those traits as well. Many of these changes have occurred as side effects of domestication in a number of species — possibly including humans…