People have wildly different intuitions about what kind of lives wild animals have and whether their lives contain more enjoyment or suffering.
I suspect that opinions about this vary a lot by how you view nature. Before the Romantic era, nature/wilderness was not seen as a charming place. Nature was what made you die of exposure or starvation.
I don’t know what people in a pre-industrial society would say if you ask what kind of life a mouse has. Maybe they’d think the question too silly to answer. But I suspect they wouldn’t have the intuition I had for most of my life, that being an animal would be kind of charming and fun.
Some of this is being raised in the era of the environmentalist movement, with its emphasis on the beauty and wonder of nature and the importance of preserving habitats so that wild animals can do their thing.
But In raising kids, I keep noticing another influence: almost all the depictions of animals they see are cute anthropomorphized ones. There are old Aesop-type animal stories with anthropomorphized animals that talk to each other, but the genre really expanded in the 20th century, starting with Beatrix Potter’s 1902 The Tale of Peter Rabbit. The illustrations make the depictions especially salient.
(There’s a whole other topic of how farms and farm animals are depicted — which is only on old-fashioned non-industrial farms run by the like of Old MacDonald — but I’ll stick to wild animals here.)
In many books the animals are just stand-ins for humans: think Goodnight Moon or The Berenstain Bears where the characters live in houses and go to school. But even the ones where animals do animal activities leave out most of the things that might be unpleasant for actual animals, like starvation or being eaten. The Very Hungry Caterpillar‘s only problem is a stomachache after eating too many pickles and cupcakes.
Another factor is that children’s books are designed to be read at bedtime, so a large portion of them end with the characters going happily to sleep. My favorite cozification of animals is Ashley Wolf’s illustration of the Raffi song “Baby Beluga”, where the (fish-eating) whales snuggle fish as we read
When it’s dark, you’re home and fed
Curl up snug in your water bed.
So naturally kids conclude that wild animals have charming, pleasant lives.
These animals aren’t living in a dirty hole getting rained on without enough to eat; they’re nice middle-class animals. And we definitely don’t talk about r-selection.
Beatrix Potter, The Tale of Two Bad Mice