Why did Hashem instruct Aharon to go and meet Moshe?
Seforno: Like a Talmid goes to greet his Rebbe.
Rashbam: In order to fulfill what Hakadosh-Baruch-Hu had told Moshe earlier (in Pasuk 14) that Aharon would come out to greet him and be happy in his heart. 1
In fact, Aharon wanted to leave Egypt of his own accord to greet Moshe - perhaps when he heard that Moshe had left Midyan, he even did so - only Hashem needed to give him directions as to where he would meet him (Ramban on Pasuk 14).
Why does the Torah see fit to add that Aharon met Moshe by the Mountain of Hashem?
Ramban: To teach us that Har Sinai is situated between Midyan and Egypt.
Who kissed whom?