Dogs have two anal glands in their rectums, which emit a strong scent. Dogs sniff each other’s butts in order to get a whiff of it. The aroma they smell gives them detailed information about the other dog.
The aroma emitted from a dog’s butt tells other dogs vital information about him. It tells his sex, health status and temperament. Therefore, on a first meeting, two unacquainted dogs know if they want to befriend each other or not. So what does that mean to us? We need to trust our dog’s instincts.
If he doesn’t like another dog, there is probably a good reason. The other dog doesn't pass the sniff test. So let nature take its course. Allow your dog choose his own friends because in the long run he nose best.