My 3 yr old springer/brittany mix (Riley) seems to be so mean to other dogs. To his young brother (8 mo. old) they play fight, but mostly play. He does put him in his place from time to time, but not enough to have to break up a fight or anything. Twice in the last week Riley has been very aggressive to other dogs. Once with a 4 mo. old shepard mix male, and today with another dog, a female. He has played with this particular dog before and been fine, today this dog came to our yard (we have a fenced yard) Riley seemed fine, wagging his tail like he wanted her to come in to play. She is a very submissive dog, so I figured it would be fine. I let her in the yard and Riley went right at her, growling, trying to take her down. She did growl, a very low growl before Riley went at her. I had to separate them and put her out of the yard. Before when she came to our house, Riley was tied up and she was free, and they played and played. Is it just different because it is a fenced yard, and Riley is trying to protect his domain? What can I do to stop him from acting this way. I want him to be friendly with other dogs, not perceived as a mean nasty dog. Our dogs don't have many opportunities to be around other dogs, is there anything we can do?