Commentary for Nazir 8:20

In other words, this man’s vow was from the outset for the sake of heaven and thus Shimon Hatzaddik was not worried that the man would eventually regret his vow. However, in other cases since the vow is made in a moment of anger or said in response to a passing sin, Shimon Hatzaddik felt that once the moment passed the person would come to regret his vow.