Accidental Friendships of Proximity

How does one make friends? It seems that so far most of my friends have started as accidental friendship of proximity. And while a few of them have flourished, the majority have dwindled.

In grad school, you have those introductory classes that at least brings the first year class together. You have large seminars, homework, shared offices and labs, etc. All of these leads to people being encouraged to talk to each other.

New faculty seem to get invited by other faculty. They also get new homes and can foster some friendship with neighbors.

So where do post-docs figure in all this. A friend of mine who graduated a year or two before me mentioned that post-doc years are the loneliest years. Why? Are we all supposed to have no life and work (or pretend to do so) 24/7?

So far I have made friends with my officemates. And I am on very friendly terms with the college admin. But I feel that these are going to start and end as accidental friendships of proximity, since we really don't have much in common...