Nice to a Stranger?

It's been already more than a year since I moved to California from Japan, and I've seen some cultural differences between the two countries since then. One of the things I've noticed is that people here seem to know how to be nice generally better than people in Japan.
Let me give you some examples.
One day, I was pushing around my 5-month-old son in a stroller on a campus. Then every time we (the baby and I) got close to a door, the students there opened the door for us! Non of them seemed to show any hesitation about helping us. I think this part is important to think about the difference between them and people in Japan. I mean, Japanese people are not heartless, of course. But from my experience, not very many Japanese people would help us without showing any hesitation in that situation.
Here is another example. I'm pretty sure that a lot of people would agree with this; people here (in the US) greet more than people in Japan. I'm talking about the situations like on a street or in an apartment complex. Here in California, you see a guy (you don't know him) in your apartment complex, and then they say things like "Hi" or "How are you doing" (except at night! They for some reason stop greeting or even looking at each other after the sun sets. I wonder if it's because they think it's dangerous to talk to a stranger at night.) Japanese people greet, but not as much as people here, especially at strangers. I don't think I would say things like "Konnichiwa" ("hi" in Japanese) to a stranger I see in my apartment complex if I lived in Japan. Actually, I feel like Japanese people used to do this more than they do now, and some of them still greet pretty often. But I just see a significant difference in how often it happens in Japan and in the US.
What I'm taking about here is totally based on my experience and very subjective observation. And also I'm not saying either people in Japan don't interact with others or people here are superficial. What I think is that people here culturally know how to be nice to strangers compared to people in Japan. I guess I could say that it shows how people here are generally more open minded and easy going.
What do you think?