Answer:
Cats' personalities and dogs' personalities are often considered calming to humans.
Step-by-step explanation:
Possessive case is added to cats and dogs -not humans- in their plural form because they refer to cats and dogs in general and possession must be marked. As a result, when nouns end in -s to express plural form, it is not necessary to indicate possession with apostrophe and-s: 's. In fact, possessive case is added just by adding apostrophe after the plural form -s.