2. Your colleagues are your competitors.

Your fellow graduate students—at least those in your discipline—are your competitors. They are your competitors for funding while you are a graduate student, because you will compete with them for teaching assistantships, research assistantships, fellowships (both internal university awards and external awards), travel grants, etc.  When university budgets are tight, as they are now, all of these things are in shorter supply and higher demand than usual.

When you graduate, you will compete with these same people for very few jobs. The more closely your work resembles that of a given graduate student peer, the more likely that you will be in direct competition with that person. So, the very people with whom close association would theoretically most benefit your own research are those who are most likely to be competing with you for the same scarce resources. This does not encourage cooperation, morale, or friendship (although these can develop in spite of the circumstances).