It is not that work friends can not be long term friends. I met one of my closest friend when we were working for a startup. There is something about big companies which prevents people from making true friends. And I think it is the collective belief that "work friends are not true friends". People don't want to share their mind freely with each other and there is good reason for that. We fear that we might speak something which is inappropriate. We are trying to be politically correct all the time. I am pretty sure I have offended people during college but that didn't not matter in long term. One can't always be just neutral or positive.