Well I foster, so once a year or so, I get a "new" puppy. I get the feeling of getting a new dog to train and love, but I am helping to train and rehab sick, unwanted or surrendered pups at the same time. It's really rewarding, but the plus is I get the new dog feeling!
Also it helps that I get Christmas cards and emails from the adoptive families. I feel like they're still here.