He who has a thousand friends has not a friend to spare,
And he who has one enemy will meet him everywhere.

Ali ibn-Abi-Talib (602 AD - 661 AD), A Hundred Sayings
Misfortune shows those who are not really friends.

Aristotle (384 BC - 322 BC), Eudemian Ethics
Without friends no one would choose to live, though he had all other goods.

Aristotle (384 BC - 322 BC), Nichomachean Ethics
A good friend can tell you what is the matter with you in a minute. He may not seem such a good friend after telling.

Arthur Brisbane, "The Book of Today"
The meeting of two personalities is like the contact of two chemical substances: if there is any reaction, both are transformed.

Carl Jung (1875 - 1961)
Friendship make prosperity more shining and lessens adversity by dividing and sharing it.

Cicero (106 BC - 43 BC), On Friendship, 44 B.C.

Nice sentences you got there, some are so true, I mean, sometimes we need a friend, but not anyone, just the one you need, nice sentences :)