Your dog has not reached sexual maturity yet. Although my dog is fine at dog parks, I do not allow her to "romp" and play with other dogs. We see other dogs in passing, but mostly stick to ourselves and play ball. The only exception is if Nisha's 'cousin' happens to be at the park....they go all out
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I am not saying your dog will not like it when he gets older; I am just saying 5 1/2 months is still a baby.