Zach totally attacked a Boxer yesterday and I am very upset with him! We were at my friend's house and her new neighbor has a 5 year old neutered fawn Boxer. Zach has been around this boxer before, always with my friend Donna, not me. Donna has told me Zach gets a little agitated with the dog,Duke, but has never gone after him. Well yesterday Donna, her 2 dogs, Zachary, the neighbor and his dog were all out front (off the leashes). Everything was fine until I walked out of the house. Of course Zach came running over to me followed by Duke. Duke was interested in what I had in my hand - a bottle of water - I let him sniff it and Zach just attacked him.

Zachary was a wild man!!! thankfully Donna was smart enough to pull Zachary by his back legs and got him off of Duke.
Why did Zach do this? Protective? How do I prevent something like this from happening again?