Don't be so hard on yourself. I know how you feel because I often get annoyed with myself because I don't react in the moment as I would have liked to. But I think that at the end of the day, the important thing is how that girl's gesture made you reflect.

It is so true that sometimes we are so immersed in our thoughts and our lives that we don't pay attention to those around us. And indeed the world is getting colder and more apathetic in general, but I also think sometimes it's a cultural thing to pass from the other. People here use to say hello to strangers if you are in a public place. If you are in a queue, people will try to engage you in conversation. And for those who are not used to it, it may even seem that people are a bit nosy, hehe.