Of course, like almost everyone, money and time. I'd want the best for my dog and I'm not in a place in my life where I can pay that much attention. Also, I have a cat, I love him to death, but he's very antisocial. I used to have two cats, and he couldn't have been happier when he became the sole prince of the apartment.
Ideally, to get a dog (Dogo Argentino, Doberdane or Rhodesian Ridgeback) I'd need a house with a large yard, in the countryside. And a wife who likes or at least supports the lifestyle, of course. I'm at least a decade away from that, and by then, who even knows what my priorities will be.
Anyway, it's not something I'd ever rush into, I've seen to many cases of pet owners way in over their heads.