Psalm 133:3  The blessing - life forever - is referenced back to Deut.28-30 where Moses reviews the terms of the covenant that Israel made with The Lord their God.   The blessings would be poured out if they were obedient to the commands that God had given.
Blessings that would "come upon you and overtake you" v2
Blessings  "in the city" or  "in the country" v3
Blessings on the "offspring of your body and the produce of your ground and the offspring of your beasts"
"Blessed shall be your basket and your kneading bowl" v5
"Blessed shall you be when you come in and blessed shall you be when you go out" v6
Blessings of security from enemies, prosperity in their work, family, nation, and every need met.

All they had to do was obey.  Choose life, He tells them!  Choose to obey and live and prosper.  Failure to make that choice would bring curses.  So severe and painful that one would never want to be out of the Word.  Yet, God knew that they would fall, He knew that they would not obey.  That we  would not know Him, see Him, hear Him, or love Him.  So, He made a way for redemption.  Jesus!
The true Life. In Him alone do we find the will to obey and the power to live.  I choose You Jesus. Come and bring Your Life into me that I might walk in the commands of God and  obey with "joy and a glad heart" ( Deut. 28:47 ) .  That I might "fear this honored and awesome name, The Lord your God".   Jesus, You are the One that brings true unity to us, Your brothers and sisters.  How we need you!  Thank You for Your great love and sacrifice for us.  I choose to follow You.