A good question.
I mean, it's not necessary. If a freakin' parrot can live for eighty years, why not a dog?
I suppose we do it because dogs are such wonderful creatures that the relationship is worth the pain that comes at the end.
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Dogs lives are short, too short, but you know that going in. You know the pain is coming, youre going to lose a dog, and theres going to be great anguish, so you live fully in the moment with her, never fail to share her joy or delight in her innocence, because you cant support the illusion that a dog can be your lifelong companion. Theres such beauty in the hard honesty of that, in accepting and giving love while always aware it comes with an unbearable price. Maybe loving dogs is a way we do penance for all the other illusions we allow ourselves and for the mistakes we make because of those illusions.
-from The Darkest Evening of the Year by Dean Koontz