My 10 month old GSD female has recently started a very disrturbing behavior and I would like any suggestions/advice. She is normally a 'fraidy cat" - not very aggressive. In fact we have two yorkies who basically run the house and she in no way is ever aggressive toward them. I take her to the dog park where we have a game of "I throw the ball she gets it. Once she has it she keeps it in her mouth and won't release it except once in a great while then I throw it and we go through the same thing again. It is as though she fixates on her ball and rarely lets go. She doesn't interact with other dogs and when she gets water she drops the ball in the bowl and drinks and never lets the ball too far out of her reach. The other day as I was putting her leash on to go home she dropped her ball. About that time another dog walked up to check her out as he had just arrived at the park. As he came up my shepherd suddenly lunged at him growling and snapping. I think she was gaurding her ball. Anyway I pulled her away and went home. A few days later we are back at the park she is walking around with the ball in her mouth fetching once in a while when a little dog comes over to check her out like all of them do when they come to the park and she lunges and growls at him. Again I think she is protecting her ball but I need to know how to stop this behavior. In the past she just ignored the other dogs or walked off and sat down away from them. The park is great because she can run after the ball when she will let me throw it and even though she doesn't interact alot with the other dogs she is around them and I think that is socializing her to some degree. I would like to continue to go there but I can't have her doing this.Any thoughts?