I own a boxer who will be 5 in october, and have just recently bought a bullmastiff when she was 10 weeks of age. The bullmastiff is now 9 months old and the boxer has been showing some out of the norm behaviors. First off when I took them to a dog park he attacked a large lab, he didn't bite the lab he just jumped on them and growled and kept going for the lab. Once I got into an open field with him and the Bullmastiff he was fine with all the dogs. He was playing and enjoying it. Then we went back home, as we were leaving there is a trail and 2 large dogs, they looked half bullmastiff half great dane were walking and he ran up to them where they began sniffing him, He let them sniff and then attacked again. He usually is very docile, he will bark at strangers and sometimes go after them to warn, but he has never bitten anyone and usually lays around the house, he is neutered. The female I plan on breeding, I don't know if he was attempting to protect the Bullmastiff or how this can be seen. The only pattern I put together was the dogs were bigger then he was, I don't know if he was showing dominance or if he has issues with larger breeds. The other thing was it was on a straight and narrow trail, I don't know if maybe it was the head on meeting of the dogs, and finally I don't know if the other dogs were male or neutered. I have read that unuttered males can be a problem for neutered males. Those are the only few things I can think of. I am not so much worried about my dog because he was able to grab on to the dog, but did no damage. Therefore I know he would not be the one that cause any serious issues, but if he jumps on the wrong dog I am afraid a serious fight might ensue and considering its larger dogs he is after, I am sure I will end up with either a hefty vet bill or a dead companion. I don't know what else to read into, lately I have been taking him with a shock collar on and I will zap him if he goes near any other dogs before we get into the field, but I just want him to be able to be open and sociable. The bullmastiff is wonderful, but he has just had some issues. If anyone has any ideas or could shed some light onto what could be causing a 4 year old neutered male to become aggressive in certain situations with other dogs, I would greatly appreciate it. Thank you in advance.