Our expert says:
Hi Dog lover
This is a strange one and my answer is "I don't know". However I can guess that it may be a type of care seeking behaviour similar to a cat kneading or suckling on humans. Perhaps he was removed from his mother too early. It could also be a sexual behaviour like masturbating, or perhaps it's a combination of the two. Neutering him may reduce the behaviour. I would certainly not worry too much about it, but perhaps discourage it by giving him a soft toy to replace your arm.
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