Letters 48.2

But the fact is, the same thing is advantageous to me which is advantageous to you; for I am not your friend unless whatever is at issue concerning you is my concern also. Friendship produces between us a partnership in all our interests. There is no such thing as good or bad fortune for the individual; we live in common. And no one can live happily who has regard to himself alone and transforms everything into a question of his own utility; you must live for your neighbour, if you would live for yourself.

Seneca, Letters to Lucilius 48.2, translated by Richard Mott Gummere