One of the things I loved about living in Manhattan was running into people I knew every time I left the house, even if I hadn't seen said friend in weeks or months. I think it happens because people are so crowded together, not just in Manhattan, but in a few specific parts of the island (Upper West Side, Village, Hell's Kitchen, etc.). Also, they tend to be on the street or in public transport.
The biggest shock about moving to LA was realizing that unless I picked up the phone and made a definite date, I wouldn't see friends at all. The city is too spread out and everyone is in a car.