As a canine behaviourist, "He's only playing" is possibly my least favourite expression in the world!
Of course, many dogs love to play with other dogs, but with an estimated 64,000 dogs being killed or injured by other dogs in the UK in one year*, it's clearly not without risk.
Good canine play is balanced. Both dogs get to have fun and enjoy themselves without feeling threatened or bullied at any point. Play can change quickly from happy and relaxed to frightening or even damaging, so it is essential that we monitor our dogs' play, and intervene where necessary.
It's simply not true that dogs should be allowed to sort it out themselves. Even if neither of the dogs are physically harmed, the emotional effects of a bad experience can last forever and can create serious behavioural problems that could have been avoided.
Good play, on the other hand, can be fun, stimulating and enjoyable for both parties - it can also create and maintain bonds between dogs that remain friends for life.