The Bible frequently records God The Father's eagerness to grant requests . Jesus teaches to pray courageously with no fear, for God will answer graciously. The parable does not suggest that prayers only get answered if the petitioner is persistent. The persistence of prayer is for the benefit of believers themselves not for God. It teaches believers should not grow weary in their prayer life, or get discouraged.
Disciples are to pray with unreasonable stubbornness until the prayer is answered.
Jesus assures His disciples God is willing to answer the petitions of His children, even when asking is left to the last hour because He was not thought of first. Jesus did not suggest that believers are to overcome God's reluctance to respond to their request. Jesus gives assurance of answered prayer. Sometimes God The Father answers the request made immediately and in other cases after prolonged asking. A 'no' answer would be that the time is not right for the petitioner's best interest.