I recently moved to a much more private apartment, but I used to be much more on top of people, and it always surprised me how little you run into your neighbors, and how much people don't bother to do anything other than say hello once in a while.
I dunno, I think Boston is an unfriendlier city than a lot of places. It brings the hate out, that's for sure. You have to actively fight it.
Really what it comes down to me for me is I constantly see people being mildly disrespectful of others, or just being rude (but pretending to be ignorant about it). This rears its head most often on commutes. I mean, I walk and take the train, and you've got the full gamut: Chinese people pretending you don't exist, people blasting music or screaming obscenities early in the morning, people sitting in handicapped seats, people blocking the doors or not letting people off a crowded train, aggressively riding their bike/skateboard/scooter through people traffic etc etc.
I won't even mention how f*cking terrible the drivers are. I throw a middle finger at least 3 times a week.
It really wears you down. At the beginning of each workweek I tell myself "this week I'm not going to get p*ssed off or react to any crappy thing that any a-hole does" and I always fail within a day. I mean, I'm basically looking for the opportunity to get angry with jerkoffs, so it's very much on me.
I'm getting very off-topic here, but city living just wears you down sometimes.