Rats, somewhat unfairly, have a reputation for being dirty creatures who carry plague. They're almost always villains when they turn up in animation, and they don't come much more villainous than the one that tried to attack John Dear's baby in 'Lady and the Tramp.'
That scene alone was quite distressing, but it's followed up by Tramp being blamed for the incident, and then send to a dog pound to be disposed of. This film isn't as bright and breezy as we all remember.