A wise man gets more use from his enemies than a fool from his friends.

[Al varón sabio más le aprovechan sus enemigos que al necio sus amigos.]

Baltasar Gracián y Morales (1601–1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 84 (1647) [tr. Jacobs (1892)]
    (Source)

Because friends flatter while the criticism of enemies can help someone correct his shortcomings. See also Aristophanes.

(Source (Spanish)). Other translations:

The wise man draws more advantage from his Enemies, than the fool does from his Friends.
[Flesher ed. (1685)]

To the wise man his enemies are more useful than his friends to the foolish one.
[tr. Duff (1877)]

To a wise man, his enemies avail him more, than to a fool, his friends.
[tr. Fischer (1937)]

The wise person finds enemies more useful than the fool finds friends.
[tr. Maurer (1992)]