Friends are people I like, share hobbies and interests with and want to have around me in my life. I picked my friends myself and I'm proud and happy with them.
Coworkers are people I'm stuck in a room with 40 hours a week. Of course you should be polite and friendly, because you're stuck with them. They got foisted on me and dealing with is part of why I get paid.
There's a huge difference between "not a living hell" and "sharing my private life and feelings". If everyone is professional and polite, that's great, but I dislike quite a lot of the people I work with and wouldn't spend 10 minutes with them if I didn't get paid for it.