My dog and my neighbors dog?

My dog is 10, and very friendly and loves everyone he meets. We have a neighbor that lives behind us, and they have a dog, also. I think their dog is around the same age as my dog, and she is a female. They are around the same size also, her being maybe a couple inches taller. My dog and my neighbors dog always loved each other. If they were both outside at the same time, they would sniff each other through the fence, playfully bark and my dog would try to dig to get to her. It was overall pretty adorable, and they never fought. Now, when my dog goes outside and she is also out there, he barely acknowledges her. He will maybe sniff her once and then walk away. No barking, no tail wagging, nothing. My dog isn't afraid of her though, because he walks past the fence she is behind, his tail isn't between his legs, he doesn't necessarily keep his distance as if he feels he is in danger. What makes this even more weird, is that she sits by the fence, waiting for him and he pretty much ignores her. They have never fought or anything, because there is a fence between them, and someone is always home when he goes out, so we would hear them if they were barking at each other. I know this isn't an urgent problem, but I'm just really curious to know what could have happened?