How could Yosef kiss his father? It is harmful to do so, due to the power of Tum'as Mes!
Ohr ha'Chayim: Tum'as Mes did not apply to Yaakov, seeing as he did not die. 1 It was as if he was sleeping.
Malbim: His body was Kadosh; it did not become Tamei via death.
Why does the Pasuk mention that Yosef fell on his father's face?
Malbim: This fulfilled Hashem's promise that "v'Yosef Yashis Yado Al Einecha" (46:4).
Ha'amek Davar: Only Yosef fell, for at the last moment of Yaakov's life, Yosef was close to him and Yaakov spoke to him covertly. Later, Yosef told his brothers, "ha'Elokim Pakod Yifkod Eschem" (50:25), a Siman of the redemption (that the redeemer will say so). How would Yosef know more than them? Yaakov had a flash of Ru'ach ha'Kodesh at his last moment, like a lamp that flashes light before it extinguishes, and Yosef saw this on the lips of his father.
Why did only Yosef cry over his father?
Ha'amek Davar: All the brothers cried bitterly over him, and also others [in Egypt] did! Also, is it honorable to kiss a Mes?! Yosef's soul was tied to Yaakov's more than all the sons. He cried amidst great love. He kissed him a parting kiss amidst crying. "Va'Yishak Lo" (as opposed to va'Yishakehu) implies that they kissed each other, like I explained above (refer to 29:13:155:1). Even though this is written after death, it means at the time of death, like it says about Rachel "b'Tzeis Nafshah Ki Mesah, va'Tikra Es Shemo" (35:18).
