This section is one of my favorites from the whole Bible. God is teaching Moses and Aaron to represent Him with self-sacrificing compassion. He Himself is going to die for people that grumble against Him, and here He is doing a little bit of that kind of training on Moses and Aaron.
A Song of Ascents. Of David.
 Behold, how good and pleasant it is
when brothers dwell in unity!
 It is like the precious oil on the head,
running down on the beard,
on the beard of Aaron,
running down on the collar of his robes!
 It is like the dew of Hermon,
which falls on the mountains of Zion!
For there the LORD has commanded the blessing,
(Psalm 133 ESV)
Fellowship, unity, brotherly peace, is like dew on Aaron’s beard! Let’s talk about that nasty example for a little bit.
When a priest or king was anointed, ancient Jews would pour oil on his head. Oil was a symbol of God’s presence and power upon a person. (I’m trying hard to not use the word SPIRIT because they didn’t use that word much in the OT when they talked about God’s presence, but it’s the same thing.)
Oil on the head was a sign of someone being set apart to be an intercessor, a priest, between God and Man. Someone that was a priest would enjoy the phenomenal privilege of interacting both with God in the Temple or the Tabernacle and people out in their daily life. This was where Heaven invaded and overlapped Earth.
Aaron was the first such priest. He was the brother of Moses and he began the line of the Levitical priesthood. When Aaron was ordained a priest, it was almost as big of a deal as the giving of the Ten Commandments or the Red Sea parting. When Aaron was made a priest, God was saying “This is how we are all going to talk to each other, and Aaron is going to go first.”
SO what is it like when brothers dwell together in unity? It’s like God has created a new way to communicate with His people, and He is establishing that way and blessing it so much that it’s dripping off a guy’s beard! God wants to interact with us so much! God wants to have a conversation, a community of people together in peace and harmony with each other!