Because (wo)man can't trust another (wo)man.
It is the nature of the Dog that Man like, You see they are an adorable little creature who develops a genuine liking towards their owner and sometimes goes to the extent to hurting intruders or attacker to defend their owner.
I have seen people getting attached to this creature so much so that they get devastated and literally go mad when they lose their dog (to death or accident), I always wonder, why, It is just a dog?
Until very recently My friend adopted a Pomeranian puppy. It was running around the house and she is very playful and watching her grow and not lose the interest in the world is one of the most amazing things. My friend who is not a dog person turned into a dog lover.
And the amount of affection these creatures shows towards their owners cannot be found in other animals (It can be found but, Come on would you prefer a dog in your house or a dolphin, Be honest :-D)