Neighbours

Yesterday, I said hi to the neighbours upstairs. Today, I said hi to another neighbour, from a house nearby. These were friendly people who wanted to greet me. Other people do not. I wrote about this earlier, it still puzzles me. What is the difference between people who want to greet a pup and those who do not? I cannot see any difference when looking at them, so I try to greet everyone. Of course it makes me disappointed when they do not want to greet me back. How can I tell in advance who wants to greet me or not?