Oh that's easy! Somebody tells you as a child that its a great idea to take small adorable creatures that instinctively love to nibble on everything around them in order to sharpen their brittle front teeth into razors and upon tasting blood or meat of any kind literally switch from herbivore into bitey carnivore mode into their home to pet, love and caress like their own little fur babies.
It also helps if a friend or say, a dumbass older brother feeds them hamburger as an experiment. But even if they don't it just takes that one explorative nibble to far to draw blood.
Ever seen Gremlins the original movie? Shit was based on real life. Pets had an existence before we took their species in as play things and there are weird things in the fine print of their DNA!
Specifically squirrels can be pretty freaky depending on the species. Castration by a very limited sort of cannibalism is a thing with some of them. This picture is right up there with putting your head in the mouth of a tamed lion...