Never injure a friend, even in jest.

Vergiss Kränkungen, doch Vergiss Freundlichkeiten nie.

Do not fear the loss of a friendship.
Anyone who is willing to end a relationship because of a reasoned difference of opinion is not worthy of your friendship.

Never do a wrong thing to make a friend — or to keep one.

Never offend an enemy in a small way.

If you don’t see the virtue of friendship and harmony, you may learn it by observing the effects of quarrels and feuds. Was any family ever so well established, any State so firmly settled, as to be beyond the reach of utter destruction from animosities and factions?

Don't carry a grudge.

Keep your friendships in repair.

there would be times when we disagreed but there would never be times when we had to be disagreeable.

Don’t let the little things divide you when your agreement on the big things should bind you.

There can't be a quarrel without two parties, and I won't be one. I will be a friend to you in spite of you. So now you know what you've got to expect

No friendship is an accident.

Be courteous to all, but intimate with few, and let those few be well tried before you give them your confidence. True friendship is a plant of slow growth, and must undergo and withstand the shocks of adversity before it is entitled to appellation.

Be slow to fall into friendship, but when you are in, continue firm and constant.