I grew up with dogs and have one now that I absolutely love. Would have never thought that I could give a dog away, but I did. It's kind of a long story:
My wife and I are huge dog people. That is, we really love dogs, not that we're huge or only love huge dogs or are some kind of dog/human hybrid mutants (I think this every time someone says they're a huge *whatever* person). Christmas of 09 thinking that I would soon be hired somewhere besides my flight instructing job, my wife said she wanted a dog that would make her feel safe at home alone, running by herself, etc. We decided on a German Shepherd and began the search. The dog we ended up choosing was a beautiful white GS from the local rescue organization. Estimated 5 years old, had been found out on the street abandoned with heartworms, malnutrition, etc. However, also had had a collar so they knew he had been someone's dog at some point. Very quiet dog. Guarded was what we thought, and it made sense considering what we knew of his history. No outward signs of aggression except that he didn't like to have his tail grabbed. Well, okay, if I was a dog I probably wouldn't like that either. Very even tempered. We live in Austin, TX and at the time lived close to downtown. Would take him out on walks and he was very well behaved. Let folks come up to pet him with no growling or hackles or anything. I even let kids pet and hug on him if their parents let them.
Things were going great until my parents came to visit for Christmas. We had had the dog for about 5 weeks and discovered that if you made kissing noises at him, he would come up to you and give you a lick on the hand or whatever you presented to him. My family (and everyone else I showed this to) thought it was a neat trick, and would do it often, with nothing out of the ordinary. I'd seen him do it dozens of times. The day after Christmas we were all seated around the table enjoying some leftovers. My mom leaned over in her chair and made the kissing noise. The dog came up to her like normal, but instead of licking, he just snapped. Without any growling, hackles, or warning at all he jumped on my mom, snarling and biting her face. It took a split second of total shock before any of us realized what was happening and I was able to jump on him and pull him off. Luckily he reverted as quickly as he had started and didn't fight me too. My mom ended up having to get 12 stitches on her face to close up the gash he left. We don't know if it was something my mom did that reminded him of some bad memory, the smell of food in the air (we had given him the bone out of the ham but it was nowhere near the table at the time), or something else entirely. And that was all we saw of it, but it was enough. We contacted the German Shepherd rescue and arranged to take him back. They assured us that he would not be adopted out to anyone else, and as far as I know is still living with the foster we left him with at a house with a lot of land and about 8 other GS dogs.
It was tough on us because even after just five weeks we had already developed a bond with this dog. Still, the memory of him snapping like that will stay with me for a long time. As bad as it was that he got my mom, I'm glad that the whole family was there to contain him and see to her. I shudder to think about what could have been if this had happened when he was home alone with my wife, or one of the several times I let a child get up close to him. Ugghh! I guess that with any dog, but especially a rescue, there are just some things that you don't understand about them and maybe never will.
The story has a happy ending though. After this whole debacle we still wanted a dog, and my wife still wanted a big dog. I put my foot down that it had to be at the bottom end of the aggression spectrum. After some additional research, we got in touch with the Houston Collie Rescue and adopted a 9 month old collie pup. It was also important to me that we could know his whole history, which they did (given up by the older woman who bought him because he had too much energy). That was one of the best decisions I've ever made. He is the sweetest, most affectionate dog I've ever seen, and even though he's pretty good sized (75lbs) he has about as much killer instinct as a wet noodle. We even had him be the ring bearer in our wedding!
So, IMHO giving up a dog doesn't mean you're a terrible person, that the dog given up will have a terrible life, or that you will never be able to have (and love) a dog. Sometimes it is just the right thing to do. Good luck!
Also, I never miss an opportunity to show this one:

