1. Housing. If you don't own a home (and in this economy, even if you've paid down 3/4ths of your mortgage, you can't count on owning your home), you are going to have trouble finding housing. I have owned dogs that range from 15lb to 90lb and even with a 15lb adult beagle mix, I had trouble. Now, with a 70lb GSD, we had to obtain special approval and we pay $100/month extra in "pet rent." At our last rental, we had to pay a $600 nonrefundable pet deposit per year. We have a GSD, but since she's a mix, our vet calls her a Belgian Mal. If it weren't for that, I don't know where we would be able to rent; GSDs are banned breeds at almost every rental community.
2. Adopt vs buy. Adopting vs purchasing a puppy from a reputable breeder is a difficult decision to make. If you choose to purchase a puppy from a reputable breeder, you're looking at a price tag of at least $500, probably closer to $1,000; but if you adopt, you may bring home a dog that has behavioral problems requiring lifelong management. (Many dogs do not exhibit their full personality in a shelter.) For me, personally, my first rescue was extremely fearful of humans and took about 2 years to rehab; my current dog (GSD mix) is 6 years old and severely dog aggressive and may never be able to be unmuzzled around another canine. That means I can never take her out, even for a 3AM pee, without muzzling, triple checking the collar and leash, etc., and we will have weekly training with a veterinary behaviorist ($$$$$$) forever.
3. Veterinary care is costly! We unknowingly adopted a dog with EPI, and she has also developed hip dysplasia. Those conditions cost us another $60+ per month, but of course we would never not treat those ailments.
4. Travel. Travel is limited to places we can reach by car, so we can take her with us; behaviorally, there is no way I could board her or leave her with a friend or sitter. However, being forced to avoid travel helps us save money!