The reason is because the Torah considers Sholom Bayis to be of the highest importance. Hashem allows his own name to be erased to restore Sholom Bayis. Hashem created men & women & the emotional needs of each one for the relationship to last. A man needs to feel that he’s number one in his wife’s eyes when it comes to honor while a woman’s nature can tolerate foregoing her honor for her father in law. A man primarily needs honor from his wife, a wife primarily needs love from her husband.
A son in law is certainly urged to forgo his honor for the honor of his father in law but ultimately he comes first.
Here are the words of Rambam Perek 15:19/20 hilchos Ishus:
Similarly, our Sages commanded that a man honor his wife more than his own person, and love her as he loves his own person. If he has financial resources, he should offer her benefits in accordance with his resources. He should not cast a superfluous measure of fear over her. He should talk with her gently, being neither sad nor angry.
“And similarly, they commanded a woman to honor her husband exceedingly and to be in awe of him. She should carry out all her deeds according to his directives, considering him to be an officer or a king. She should follow the desires of his heart and shun everything that he disdains.
This is the custom of holy and pure Jewish women and men in their marriages. And these ways will make their marriage pleasant and praiseworthy.”
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