Summary: Be wary of friends - they will betray you more quickly, for they are easily aroused to envy. They also become spoiled and tyrannical. But hire a former enemy and he will be more loyal than a friend, because he has more to prove. In fact, you have more to fear from friends than from enemies. If you have no enemies, find a way to make them. - Robert Greene

Longform Explanation: Trusting friends is the easiest way to experience downfall, especially when in a position of power. We know them personally and trust them more than random strangers. They often hide who they indeed are for the sake of their friends, electing to conceal the devious parts of themselves.

What’s the problem, then? You hired them. You are bringing resentment and jealousy out of them. Not every time, but this can happen, and one must be on guard to rectify the situation.

Proof? People want to feel they deserve any good fortune, not just giving it to them. By choosing them because they are your friend, you are doing the opposite, giving them something simply because you know them. Over time, this seed of kindness will grow into a small plant of resentment and jealousy, watered by your compassion.  The best thing to do is not expect any gratitude; that way, when you receive it, it comes as a pleasant surprise.

Instead of hiring a friend, hiring someone who excels at the task is more important and necessary. Prioritize competence whenever possible, and work to keep friendships separate from business.

Your enemies, on the other hand, are an untapped gold mine that you must learn to exploit. - Robert Greene

Recognize the opportunity you make when you decide to turn an enemy into an ally.  Enemies have no reason to trust or believe you, so you might have made a more trustworthy ally than a friend when you provide a favor.  After all, the former enemy owes you one.

Not all cases require you to turn an enemy into an ally. We often become sloppy and unkempt, comfortable in our genius and the hail of praise. Having enemies causes you to stay on your feet, knowing an adversary is waiting for your mistakes. Understand that you will see no reason to improve or keep your skills sharp without a worthy adversary.

Three points: 

1. Never purposely pick a fight with someone you’re not sure you can beat.  Ensure winning is inevitable, even if the battle draws itself out. 

2. If you have no enemies, it might be in your best interest to create one if they are just superficial enemies.

3. Use enemies to clarify your positions to anyone, especially those who may be confused about where you stand.

Finally, do not ever let the presence of the enemy distress or upset you.  You know who they are and do not have to infer intent like you would with a so-called friend.  You know the purpose of an enemy, and their presence can strengthen your resolve.

Reversal:

Generally, it is best not to mix business with friendship; however, there are times when having friends around is beneficial.  Scapegoating a friend can be effective, as those looking on will not believe that a friend of yours is capable of doing the things mentioned. Know that once you scapegoat a friend, you’ve lost them forever, so only use this as a last resort, or use someone you are okay with removing from your circle. Friends can be significant assets, but remember not to trust their motives fully; respect and rely on your senses when you notice something about your friend.  Lower your guard, and you could lose everything you’ve worked to obtain.