I read something once about the difference between friends and associates, and it said an associate is someone you have a paid relationship with, and you might think they're a friend, but once the paid relationship goes away you usually find they were associates. Sounds like that's what you've got.
Also, I hate to say this, but there are a couple of people who left my office in the last few years who I will avoid like the plague now that I can, even though I was always friendly and chatty when I worked with them.