If you could keep your friend, approach him with a telescope, never with the microscope.
Anon
Give not thy friend so much power that if one day he should become a foe, thou mayst not be able to resist him.
Saadi
Don't flatter yourselves that friendship authorises you to say disagreeable things to your intimates. On the contrary, the nearer you come into relation with a person, the more necessary do tact and courtesy become.
Oliver Wendell Holmes
Keep your undrest, familiar style for strangers, but respect your friend.
Coventry Patmore