You have friends and enemies.  Friends are nice to you and enemies are mean to you.  There is always a chance that a friend turns into an enemy and an enemy turns into a friend.  An enemy is more likely to turn into a friend the nicer you are to him.  A friend is more likely to turn into an enemy the meaner you are to him.

If your enemy is your sworn enemy, hating you whatever you do, there is no point being nice for strategic reasons.  But the fact that you want to persuade an enemy to become your friend means you have a greater incentive to be nice. So, Gordon Brown should have gone to Rebekah Brook’s wedding and Sarah Brown should have invited Rebekah over for a pyjama party.

The same logic applies to your friends: be nice to them to persuade them to stay your friends.  But don’t be too nice because that may backfire when they become your enemy.  This is particularly important in terms of giving them incriminating information they can use against you once they turn on you.  If you are prone to disappointment, it applies more broadly.  Woodrow Wyatt who helped his friend Rupert Murdoch evade regulation when he bought The Times found this out when Murdoch chose to endorse Blair:

Source: This excellent life of Murdoch in Britain by the BBC’s Alan Curtis