I spent a very significant amount of my waking time working. This is one of the best opportunities to make friends, and IMHO it's also nicer to work with friends than with non-friends.
Most of the friends I made as an adult came from a work related setting.
I can build relationships of varying degree of strength in and out of work.
When you inject friendship in a work relationship, you are also injecting mechanics that are not work-related and can ultimately lead to problems in the work setting.
You can be pleasant around co-workers but being all out friends is dangerous.