Studies suggest that puppies respond more readily than the adult dogs when people talk to them they respond like the babies do- more slowly but with a higher tone. When talking to dogs, human adults use pet-directed speech similar to infant-directed speech which is known to engage infant attention and promote language learning.
Researchers led by Nicolas Mathevon of Hunter College, the City University of New York in the US showed that puppies are highly reactive to dog-directed speech but that older dogs do not react differently to dog-directed speech compared to normal speech.
It remains an open question whether puppies react innately to dog-directed speech and exactly why adult dogs showed a lack of preferential reactivity to dog-directed speech. People seem to consider dogs non-verbal companions and speak to them as they would to human infants.
We use similar strategies in other situations where we believe our listener may not fully understand us, such as when speaking to elderly people or linguistic foreigners.