I remember I really upset some coworkers a few years ago when I said coworkers can't be my friends. I made it clear that I like a lot of my coworkers, I think most of them are decent people and are OK to talk to, but that's a bit different than being a "friend".

A "friend", to me, is someone I can completely be myself around without having to worry too much about it [1]. With a coworker, in the back of my mind I am always remembering "I have to work with this person tomorrow, best not talk about that subject..." and have to bite my tongue a bit. I don't just go around spouting racial slurs or anything too crazy, but my coworkers are exposed to a "tombert-lite" all the same.

Also, if you work for startups and if you become best friends with all your coworkers, when the startup fails [2] then you can find yourself not only out of the job, but also hating a large percentage of your friends in the process.

Now, I've quit/been-fired-from jobs and kept in touch with former coworkers, and then I consider them friends, but I do try and draw boundaries for the people I'm actually working with.

[1] Within some degree of reason, obviously.

[2] Not all startups fail, but an awful lot of them do.