Why do people get a dog, and then give them up?

I was asked this question at a dog park recently.  Why do people get a dog, and them give them up?  The answer is probably due to poorly set expectations.  Common "free to a good home" adds state - "not enough time" "needs a bigger yard" or "moving and can't keep" - these things all come down to people not realising that raising a dog takes time, and effort. You can't really stick them in the back yard - not interact with them and expect them to be well-behaved.  Having a dog that is well behave might take some effort - but it is not hard.  A simple understanding of reinforcement and how it influences behaviour is all you really need to build an everlasting relationship with your dog.