Millions of dogs are euthanized each year. The main reason is because there simply isn't enough room for all these dogs. Currently there is just way too many dogs reproducing which is resulting in this overpopulation. A breeding pair is going to have upwards of 8 puppies. Each of those 8 puppies could potentially create 8 puppies of their own one day and it just spirals out of control. Before you know it there are way too many dogs, and no where for them to go. (which is our current situation.)
So how is it decided which ones get a chance and which ones do not?
Firstly, in a perfect world every dog would get a chance. But this isn't a perfect world and the over population problem is real. This means that it is the best interest of shelters and rescues to try to adopt out the dogs with little to no problems. These dogs will get adopted quicker which will open up a space for another dog.
This leaves the dogs with more serious issues left. These more serious issues typically come in the form of some sort of aggression. (e.g. towards people or other dogs.) If a dog is showing signs of aggression towards people, a shelter can not adopt that dog out in that current state because it's a ticking time bomb waiting to happen and someone is going to get hurt. This means there are a couple options for these dogs. One option is to do some behavior modification. The problem is that true behavior modification takes months and months. (It took me nearly a year to get my dog over his issues with other dogs.) If it is going to take months and months to rehab a dog to get him adopted, this is going to result in that dog taking a spot that many dogs could have used to be adopted. It goes back to the dogs with "no issues." A dog with "no issues" is going to be adopted out quicker. Within the time that the dog with issues needs for its rehab 10 + dogs could be adopted. If the dog with issues is taking that spot for that long, it's going to result in 10 + dogs being euthanized because there is no room for them. It really is crappy situation. Another option is for a rescue to take in a dog with these issues and keep him in a cage for the rest of his life. (Is this fair?) And finally, the other option is euthanasia. Once again, no one gets into this line of work to put dogs to sleep. But the reality of the situation is that there simply isn't enough room for all the dogs in this country and this is going to be the end result for many of them.
It is my goal by writing this to help people understand that no one wants to put a dog to sleep. No one signs up for this job to kill dogs. People that work with dogs do it because they love dogs. There simply isn't enough time, room, or resources to save every dog. Additionally, not every dog with behavioral issues can be, "fixed." The general public has been mislead in regards to what is possible with behavior modification. Dogs are living, breathing animals that can't just be changed by snapping ones fingers.