And you will then return to the L‑rd your G‑d, and you will heed His voice, according to all that I command you today. You and your children [will repent] will all of your heart and with all of your soul. In the place where the ba’al teshuva stands, complete Tzaddikim cannot stand.³ 34b. The greatness of the ba’al teshuvalies in his familiarity with the heat and passion of sin, for when he does teshuva and applies himself to Torah study and mitzvos, he does so with great enthusiasm and passion. He remembers well the fervor, which is not the case of the Tzaddik, who has never experienced this in his life.

footnotes: ³ Berachos

My Master and Teacher (the Baal Shem Tov) offered another reason. Good deeds and Torah study can become routine for a Tzaddik, and he does them without much enthusiasm. However, they are new for a ba’al teshuva. Furthermore, if the ba’al teshuva does not do everything with devekut, he will fall from his level. Therefore, he must serve his Creator with great enthusiasm. Ohr Torah, Toldos