Greater love has no one than this, that one lay down his life for his friends. John 15:13 Reina-Valera 1960 (RVR1960)

A treasure is something clearly valuable, important and greedy that we all take care of with the greatest caution and prudence, to keep it in a very sacred place that is not available to other people, apart from its owner.

The love of Jesus should be regarded as the highest and most excellent treasure that every pearl and diamond seeker should yearn for. The love of Jesus surpasses all earthly treasures that anyone can covet.
Only when someone can lay down his life for another does Jesus say that the glory of true love is manifested in that action.
Of course, Jesus was speaking to his disciples of what was going to be like in the future for them and for all mankind.

If someone is going to give their life for a friend, they would do so for the compassion and tenderness that binds them to that person. What unites, really is love. Jesus was not only willing to give his life for the love that united him to his disciples, but for the majestic and great love that the heavenly Father has for all humanity.

For God so loved the world that he gave his only begotten Son, so that everyone who believes in him may not perish, but may have eternal life. John 3:16 Reina-Valera 1960 (RVR1960)

Truly, when God gave His Son's life for us, we find ourselves deprived of God's glory, steeped in sin and rebellion. In other words, we were enemies of God, however, God preferred to give the life of Jesus, for us, even though we did not deserve it.

Jesus had to give his life for the enemies of God, so that in his love, we would be made friends and children of God the Father. How great has been the action of the valuable treasure of the love of Christ, which has given us life being dead in sin and has made us children of the Most High and sovereign God.

Why is the love of Christ more valuable and excellent than any other treasure? Simply because in his love he brought to God the most valuable treasure that had been lost in the Garden of Eden and that he had now recovered, giving his life to win multitudes of children for his heavenly Father.