What can be more delightful than to have someone to whom you can say everything with the same absolute confidence as to yourself? Is not prosperity robbed of half its value if you have no one to share your joy?

[Quid dulcius quam habere quicum omnia audeas sic loqui ut tecum? Qui esset tantus fructus in prosperis rebus, nisi haberes, qui illis aeque ac tu ipse gauderet?]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
“Laelius De Amicitia [Laelius on Friendship],” ch. 6 / sec. 22 (44 BC) [tr. Shuckburgh (1909)]
    (Source)

Original Latin. Peabody (below) attributes the first sentence here to Ennius, whom Cicero quotes in the previous sentence, but nobody else does. Alternate translations:

What can be more delightful than to have one to whom you can speak on all subjects just as to yourself? Where would be the great enjoyment in prosperity if you had not one to rejoice in it equally with yourself?
[tr. Edmonds (1871)]

What sweeter joy than in the kindred soul, whose converse differs not from self-communion? How could you have full enjoyment of prosperity, unless with one whose pleasure in it was equal to your own?
[tr. Peabody (1887)]

What is sweeter than to have someone with whom you may dare discuss anything as if you were communing with yourself? How could your enjoyment in times of prosperity be so great if you did not have someone whose joy in them would be equal to your own?
[tr. Falconer (1923)]

What is sweeter than to have someone with whom you dare to discuss everything, as if with yourself? How could there be great joy in prosperous things, if you did not have someone who would enjoy them equally much as you yourself?
[Source]