In Iceland, I lived next door to a lovely couple for almost four years. I would occasionally chat to the husband or the wife if they were out front, but it never got more neighborly than that. I tried once to see if they would keep an eye on my apartment when I was leaving for a few weeks, but either my Icelandic wasn't good enough or my request was too subtle: they assumed I was just giving them some basic information. So when I moved out of my apartment, I did not make a special effort to say goodbye to them.
I only lived in my apartment in Berkeley for 8 months, but I developed a neighborly report with the couple that lived downstairs. We bonded pretty quickly, because they had a new puppy Palmer liked to try to pet everytime we saw Liz or Colin taking the dog out for a walk. And we had regular discussions complaining about the landlords. We were inching towards friendship sometime in June, after they went out of town for a week, and Liz asked me to watch her cat, Eva. After that there were some discussions about having a glass of wine on their patio with them some evening. I moved out before we had a chance to make that happen, but anyhow, they were a nice friendly couple, and they are on my list of good recent contacts I have made here in California.
Anyhow, I would hardly say this is a universal California/Icelandic dichotomy, since in most instances Californian's are not particularly friendly neighbors. Nice to find an occasional exception to the rule.