I have found this to be true during my experience living in the Bay Area as an adult for the last 23 years. In particular, although living in the Bay Area has allowed me to meet extremely wonderful people who enrich my life, I have also met many worthless people who seem to drag down everyone who gets mixed up with them. "Dangerous" is probably not the best word for getting mixed up with them: "costly" is probably better. It has been my experience that most of the "costly" relationships I have had are with neighbors and only one of the beneficial relationships I have had has been with a neighbor -- and that relationship was vastly less beneficial than many of the relationships I have has with non-neighbors.
The reason it turned out this way is not hard to explain: unless you are a landlord or are buying a house and participate in a "cohousing" venture, there is almost no way in American to choose your neigbhors, and it is difficult and expensive to put distance between yourself and a neighbor (because moving is difficult and expensive).
Note that a large part of human communication is "signalling" by which I mean making oneself appear as virtuous as possible and making oneself appear to have the character traits that people want in a friend or a coworker.
And note that the way I would signal virtue or good character traits in this conversational thread is to say that I always get along with everyone and that I love all my neighbors.
But I did not do that. Instead, I intentionally chose to risk leaving the (false, IMHO) impression that I might be hard to get along with in order to counteract what I see as the overly rosy picture painted by our society about the value of warm relationships with one's neighbors (relative to warm relationships with non-neighbors) -- and in order to give some of the younger people reading this the benefit of my life experience. (I am 49.)
It is of course often extremely beneficial to live with someone who loves you, I should add -- and that is worth going for because it is much less expensive to choose your housemates or apartment mates than to choose your neighbors.