That was a pretty dominant move on your dog's part. It isn't quite the same as aggression, but he did want to dominate her. The fact she still wanted to play says she was not scared or "offended" by his move, but recognized it as dominance. If he was being truly aggressive mostly likely she would have backed off and wanted nothing to do with him.
If you had both dogs on leash, that can also create frustration and cause him to over react. It is ideal to do introductions off leash in a fenced area, though I know that is not always practical when meeting dogs while you are out walking.
Another factor is once you know he may act like that you will tense up when you approach another dog, and that tension will cause him to lash out. I would suggest you work with a knowledgable behaviorist who can assess you and your dog in person, and can tailor a training program to both of you.
Answered by paws4hisglory - 5/16/2012 10:18:21 AM