You're probably familiar with the “brother from another mother, sister from another mister” concept regarding people you meet in life who look like they could be your next-of-kin...or you.
This spooky similarity often goes beyond merely looking the same, as you discover you share interests, taste and mannerisms in common with your doppelganger.
The same thing can happen with animals, only their “brother from another mother” similarities can cross species, making a dog and a cat look like they were separated at birth.
Perhaps this doppelganger effect exists so every animal species on Earth will learn to get along with each other, whether we're hairless apes or hairless critters.