A person born a tzaddik can progress until he attains perfection, when he completely transforms his nature from evil to good.
This wondrous phenomenon, the transformation of bitter to sweet, of dark to light, pleases God.
The tzaddik changes both himself and everything around him.
Whatever he touches is transformed to sweetness and light.
Just as whatever King Midas touched turned to gold, for the tzaddik everything changes to good.
The Talmudic statement that "a tzaddik who experiences good is a complete tzaddik" (Berachot 7a) does not mean that a complete tzaddik never experiences challenging times.
However, such difficulties do not cause him to suffer; he does not feel the evil in them.
In this spirit, the story is told about two Torah scholars who asked the great Maggid of Mezherich about the Talmudic dictum that "a person must bless God for the evil just as he blesses Him for the good" (Berachot 60b).
The Maggid sent them to his student, Rabbi Zusha of Anipoli.
Throughout his life, Rabbi Zusha lived in dire poverty.
But when these two men came to him with their question, he replied, "I don't understand why the rebbe sent you to me. I've never had a bad moment in my life."
–Rabbi Adin Steinsaltz
From Understanding the Tanya by Rabbi Adin Steinsaltz