Just got onto this thread and I'd like to add that not only do people not understand the costs money wise, but the time itself to take to train a dog.
Hate to use this example, but I'm going to anyway, my brother got a dog when he was going through his divorce because he was longing companionship, and Mr. Mac, who is a Cairn Terrier, is basically my brother in Cairn clothing personality wise. However, when my brother got a girlfriend, training with Mac went to the wasteside. So, bascially, at the age of three, Mac still can't be trusted to no potty in the house, my brother thinks its funny when he lifts his leg outside and basically never corrected it, and there have been three paniced phone calls from my brother saying this dog is runinig his marriage and that I need to take him... (My sister-in-law really resents Mr Mac).

With that being said, I don't think many people understand the physical demand with dogs. You're not just given a puppy that's potty trianed, loves to go on car rides, and "comes when you call him." Please don't think my brother is abusive by any means, he just wasn't (and isn't still to this day) prepared. I can't keep count how many shelters that I visit that the reason the dog was turned in was because the owner couldn't get over the fact that the dog was locked in a crate for 8 hours and went potty everywhere and they couldn't get him or her potty trained...