I have been a small-animal veterinarian for almost 17 years. I have seen a lot of sickness and death and, unfortunately, have had to euthanize countless animals over the years.
I was recently asked to help a local shelter because they were having a certification problem. Basically, they needed a veterinarian on the premises while they euthanized unwanted, sick or unadoptable animals.

When I arrived at the shelter, I went outside to look at the dogs in the kennels. I was greeted by a frenzy of barking and jumping. I saw a guy with a red leash walking several dogs past me, around a wall and out of sight.

It quickly dawned on me what he was doing. He was taking these dogs, one by one, to be euthanized. The looks on their faces showed that they knew it, too. Curiosity. Hesitancy. Suspicion. Dread.
It was a horrific parade that went on for over an hour. I was shocked at the number and type of dogs walking past me. A perfectly healthy-looking, shiny red dachshund. A nice, purebred boxer, a handsome black Lab. A cute, terrier puppy that kept trying to jump up on the guy with the red leash and lick his face. He reached down and petted the dog briefly, a vaguely tragic look on his face.
The dogs still in the kennels would bark louder as they went by, then settle down again. There was a faint odor of death emanating from behind the wall, where the freezer was kept, where the animals went after they were put to sleep.
After a while, the two women who were doing the euthanasia came out from behind the wall. One grabbed the hand of the guy with the red leash and they started waltzing with no music in the aisles between the rows of kennels.
It was a strange sight. I later asked what they were doing, and she replied, "What we just had to do was so horrible, we were just trying to get our minds off of it." Animal overpopulation affects the shelter workers, too.
You have no idea how horrible that was to watch.
What is the answer to all of this? It's so easy to blame the shelters for this problem, but I have come to the conclusion that this is everyone's problem. Spaying and neutering is the only solution to this problem. Period.
The offspring of every pet that is allowed to breed displaces pets in the shelter that could have been adopted.
It's just not fair to die because no one wants you.
It's time to take more drastic measures to control animal overpopulation.

We've been to the moon; surely we can come up with something better than killing animals for space.