הלכות קריאת שמע ד
ויבך על צואריו עוד וכו'
Our Sages explain that at the time when Yosef was crying on his father's neck, Yaakov was saying Shema. The Rebbe explains the secret of this incident in the 36th lesson of Likutei Moharan. In short, that when one recites Shema with tears he subdues all sexual desire completely. Therefore, one who unfortunately has such thoughts regularly must cry during the time that he recites Shema. This is why Yosef cried as Yaakov recited Krias Shema. Being the paradigm of sexual purity and the ultimate symbol of shmiras habris via the sefirah of yesod, Yosef wanted to nullify the inherent sexual desire in man, which is the culmination of all evil.