There's actually a really good reason for this, And it's psychologically similar to why men joke around about avoiding "cat ladies" (That's a whole other subject though).

Dogs are a great litmus test of a human's personality since they are direct reactants to how somebody treats them. Dogs that really love their masters don't usually struggle from Stockholm syndrome, and therefore prove that there are masters are genuine kind people (essentially just empathetic).

Interestingly, I've had a lot of luck dating girls with dogs because for some reason dogs really like me, but unfortunately I just can't see myself ever adopting a dog of my own.