How can a young man reconcile his desire to marry a girl he loves with his mother's rejection of her without a compelling reason, while also being keen not to disobey his mother?

If the mother rejects a fiancée due to her righteousness and uprightness, then the questioner should advise his mother and strive to convince her. If she is not convinced, he may marry her while continuing to honor his mother and treat her kindly, without abandoning her. However, if the mother's rejection is for another reason, then he is advised to obey her, for there is good in obeying her. And whoever gives up something for the sake of Allah, Allah will compensate him with something better, for the Prophet, peace and blessings be upon him, said: "Your mother, then your mother, then your mother, then your father."