I honestly couldn't let go either but I'm not a breeder and don't know whats all involved. Maybe because they feel they don't have the quality time and feel that they should go to a forever home so they get the attention and love they really deserve? If this is the case I really do respect them for doing this because they know whats best for the dog.
But this is a great topic and would also love to know how can they do it. I think I would have a farm of furkids because I just couldn't let go.
