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Joe remembered every single one of the animals Nicky had taken with him home because they needed to be rescued from cruel people and the world or were injured. And there had been many over the centuries. From the cute and fluffy ones to those who were slimy, poisonous, or had sharp teeth.
Every time, Nicky says, "But they also deserve to be rescued by us as well as humanity does! We shouldn't distinguish one from the other."
And every time Joe helped Nicky because, it reminded Joe of one of the reasons why he loves Nicky - namely, his strong determination to help and be kind towards everyone on Earth, from humans to animals.
And every time a human or an animal dies because Nicky couldn't prevent it, Joe comforts Nicky by just being there, holding Nicky and letting him cry out.
And when Nicky is done crying, Joe tells Nicky that he will save the next one.
Nicky's kindness reminds Joe that he needs to be kind towards others and help them no matter who they are, which is one of the reasons why Joe loves Nicky so much.