My name is Bear!

Bear is a great dog, I have had him for 8 years now. He is 10-11 years old. My girlfriend has a dog, who has lived with us for two years now. This last week he went after the dog and showed some aggressive behavior towards him. He has done this 3-4 times over a two year span. I am not sure what is causing this and feel like training at his age is not what I am wanting to do. He has NEVER shown any aggression towards people or kids, as I have nieces and nephews who love him and spend time with him. I feel he would be best with a house with no pets. He likes to go for walks and car rides. He is pretty chill and will not take a ton of energy to care for him. I will only let him go to a great home, I care a lot about this dog and its hurts to even think about him being with someone else, but it is for the best for him.