Friendship can create entitlement. Friends assume closeness will excuse mistakes, and they may resent your success as if it were a personal slight.
Judge by incentives under pressure, not by warmth in easy times. Keep friends, but do not hand them your most fragile leverage: money, secrets, access, or reputation. Familiarity breeds carelessness, and carelessness creates damage.
An enemy who becomes useful often has something to prove. Their reliability can be stronger because it is measured, not assumed. The point is not cynicism. It is structure. Trust should be earned through repeated behavior, not gifted through history. Keep your agreements clear, keep your boundaries visible, and let actions do the persuading.