Colleague or friend?

I have one colleague who always use this phrase: "We can only be colleagues, and not friends." She told this to everyone.. even to those closer colleagues.. and I sort of agree to it now..

I asked another colleague just now whether we can have friends in office.. and he said that we can have friends but it is just another category of friends.. different from those we have outside.. why and how different? he just say that certain things we can say to outside friends but not to office friends.. yup.. backstabbing can so easily occur in office.. even a slip of the mouth may cause some harm.. so everyone is guarded..

just like in uni.. we have many categories of friends.. lowest level is hi-bye friends whom our conversation only involve "Hi! How are you? Got to go.. Bye! Cya!!" then course friends whom we say more than a hi and bye.. will talk about our own course.. then closer study gang or friends whom we will take many modules together and discuss more than studies and then our close friends whom we will go graduation trip with and even maintain contact now after graduation...

Office friends just end on the level of course friends or the higher level will be study gang but is just those whom we will not talk less of personal things.. and these kind of friends are quite rare.. I know of friends who has even closer friends in workplace like those working in labs or places where most of their colleagues are about the same age as them.. lucky for them.. but not for me..

in my office.. I still prefer the phrase "colleagues" to call most people and not "friends".. like I the way i treat course friends as course mates or classmates.. "friends" is reserved for those one or two whom i see worth..