According to companion animal psychology, there are 4 theories as to why dogs play.
One theory is that play helps puppies learn motor skills. If you look at what dogs do in play, they chase each other, roll around on the floor in play fights, mount, pick up objects with their mouth and tug, bite or shake them. Puppies learn how hard they can bite their playmates (acquired bite inhibition), and to play bow to keep the play going for longer.
Through these play activities, they are learning real skills relevant to how to move their bodies, acquire food, and defend themselves in fights. The scientists say this theory explains a lot of things about play, but is not the full story.
Another theory is that play is training for unexpected things to happen: it’s through play that dogs know how to right their bodies when knocked off balance and how to cope when something surprising startles them. According to this theory, changes in the brain and in hormone levels during play help dogs learn how to cope with real-life stressors.
This theory explains the fact that dogs like new toys but are cautious of new things that aren’t toys. It also explains the way dogs self-handicap during play and put themselves at a disadvantage; this can be seen as practising behaviour they may need later on as a way to defuse real aggression. But again, this theory only explains some aspects of play.
The third theory they found evidence for is the idea that play promotes social cohesion between dogs. Play helps dogs cooperate as a group, and is about building social relationships – in which humans also feature. Dogs prefer to play with people they know, and they are more likely to approach the winner of a game, but when they win a game against a person it does not lead to increased ‘dominance’. So play is about building cooperative relationships, not social rank. But again, this theory does not explain everything about play.
The fourth theory the scientists considered is that play is just a side-effect of other processes, such as having too much energy or a deprived environment that does not provide stimulation. However, poor environments are linked with the development of stereotypies (repetitive behaviours), rather than play. If play was linked to too much energy, then playfulness wouldn’t be a consistent trait in dogs. Because play is something humans like, it may have been selected for in domestication or have arisen as a result of breeding for other traits, such as neotenic (baby-like) features. But play does not seem to just be a by-product of other things.