I think it's important to make the distinction that while you can be friends with your colleagues it's very important to also have friends outside of work. Having friends from work is great, but if you only have friends from work then you have nothing outside of work which isn't healthy (for example, if things start going poorly at work then it can feel like you are trapped).
I disagree. At any of my previous jobs when work started going poorly, my friendships were, if anything, a boon to me because I had a support system that immediately understood my work situation and challenges. They were immediately in a position to help because of their close proximity both to me and the challenges at work I was facing. After leaving one job, the relationships with my "work friends" were invaluable to me in helping me find another job since they were in the same field and had connections I could leverage.
Outside of work, my "work friends" and I go out to eat, go play sports together, hang out at each other's houses, etc. We talk about work sometimes because it's obviously a common interest, but our friendship doesn't revolve around it, and if someone severs their relationship with the employer that we have in common, it does not mean our friendship also severs.
