I’ve always heard how Yosef would look into the mirror and that he was into his appearances, and I always wondered why this was. Then I finally saw a reason.
The Torah calls him a boy, even though he was already seventeen years old, because he behaved in ways characteristic of youth.
Why did Yoseph behave in ways which were characteristic of youth? Yoseph was the child of Yaakov's beloved wife, Rachel. Everyone knew that she was the one Yaakov considered to be his true wife. This gave Yoseph the status of being the favored son among all the others. He was the son who was supposed to take his father's place.
Everyone knew that only one son of Avraham would take his place, and that was Yitzchak. Similarly, only one son of Yitzchak took his place, and that was Yaakov. Thus, everyone assumed that only one of the many sons of Yaakov would take their father's place. Since Yoseph was the son of Yaakov's beloved wife, he was the natural choice. This situation was a trial for Yoseph. He was not an only righteous son as his father and grandfather had been. They had no competition, since Yishmael and Esav were not really candidates for taking their fathers' place. But Yoseph did have real competition. All his brothers were spiritual giants. To ward off their jealousy,(10) Yoseph played the simpleton. He pretended that he was not as interested in spirituality as was his father. He preferred combing his hair, accentuating his eyes, and paying great attention to his appearance. When the brothers saw him act in this way they left him alone, thinking that such a simpleton would never pose a challenge to them in taking their father's place.
We learn from here to not show off, and sometimes to even pretend you’re lower than you are, and it will make it easier for you to become a greater person without worrying about the competition interfering and trying to bring you down. So that is why Yosef was all into his appearances, it was a show for his brothers so that they wouldn’t view him as competition and then he would be able to take over his father’s place.
Speaking about appearances, Leora writes about Tamar, how she got dressed up and seduced Yehuda and then got pregnant. And on the flip side how she was modest in Yehuda’s house so that he didn’t even recognize her.