by Rabbi Michael Unger

Shmos ch 4 verse 27

And G-d said to Aaron go towards Moshe in the wilderness and he went and met him at the Mountain of G-d and kissed him.

Ramban, ibid:

And he kissed him - Aaron kissed Moshe, because Moshe was humble and honored his older brother. Therefore the Torah did not say that they kissed each other.

The Torah did not tell us who kissed the other when Moshe and Aaron met in the wilderness. We do know that they did not both kiss each other because the word "kiss" is singular and not plural. The Ramban tells us that it was Aaron who kissed Moshe since Aaron was the older brother and Moshe was humble before him.

Even though I am not sure that I understand what honor there is in having the older brother give the kiss, I do feel that we can learn a great deal about the
humility of Moshe from this act.

It is the nature of people who are "important" to become haughty. The haughtiness that results shows up in many areas of daily conduct. The haughty speak different and act different than the rest of us. But they also tend to "forget" how they should interact with family members and previous friends and neighbors.

Moshe Rabbenu lived in Pharoah's house and grew up differently than the rest of his family and indeed the rest of the Jewish people. Opon leaving Egypt and
moving to MIDYAN, he spoke to G-d about being the leader of the Jewish nation.

It would have been understandable had Moshe forgotten the law of honoring his older brother and kissed him. But Moshe was so humble that all his importance had no effect on him at all. He remembered who he must honor and how that honor is to be given.

The Seforno adds a dimension to Aaron's kiss: He comments that Aaron kissed Moshe as one would kiss a holy object. In the Egypt of oppression, slavery and murder Aaron was sensitive to holiness to such a point that he kissed his younger brother as he would a Torah scroll.