I always have had animals, pets, and can't imagine life without them, and therefore, can totally understand how someone who never has had them couldn't imagine life with them. And really, having a dog isn't for everyone, and it might not be all that your wife has imagined. You didn't mention if she previously had dogs, at least, I don't recall seeing a mention of that.
Obviously, this is something serious that the two of you need to discuss in depth, you should really get to know why it is that she wants a dog, and so should she. Maybe she could volunteer at the local humane society, or pet sit for someone so that she can get to know all of what having a dog entails, you know, the good and the bad. Maybe she is lonely at times, or maybe she is thinking that a dog would provide security, or a reason to exercise, or maybe the novelty of it all.