One way to prove the claim is to read the obituaries of both animals and people. Here’s one for a cat from March/April 2016 of Best Friends magazine, by Carmela Hayslett:
Sailor, our time with you was far too short. All you ever wanted from us was our love, and no amount of love we gave was ever too much for you. Everyone who met you loved you dearly. Thank you for letting us be your parents and teaching us what it truly means to love so unconditionally. You will always be our little girl.
Human obituaries never come this close to love. You read about how the person had a smile that always lit up a room, or was a whiz at some video game, or into cars, etc. etc. One we once saw gave thanks that the dead guy held on a day longer and got to see a big University of Utah football game on TV.
Another way of demonstrating that animals are more lovable is to imagine a cat, dog, or horse with a human ego, posturing, pouting, primping, sulking, trying to impress, showing fits of jealousy, angling to be the center of attention. Rather than lovable it would be disgusting.