Rejoice: Feel or show great delight.
Give Praise: the expression of respect and gratitude as an act of worship.
What was the cause of such an expression? Nephi saw a day in which the following events were set to occur. (1Nephi 15)
The fulness of the gospel of the Messiah was going to come unto the Gentiles, and from the Gentiles unto the remnant of his seed (13).
The remnant of his seed was to know that they are of the house of Israel (14).
They will know and come to the knowledge of their forefathers, and also to the knowledge of the gospel of their Redeemer and the very points of his doctrine, that they may know how to come unto him and be saved (14).
They will receive the strength and nourishment from the true vine (15).
They will come unto the true fold of God (15).
All the kindreds of the earth will be blessed (18).
The restoration of the house of Israel in the latter days at which time they will not be confounded nor scattered again (20).
Anyone of these could be written about for pages but my favorite and the one which I rejoice and give praise unto my everlasting God for is that I can receive strength and nourishment from the true vine.
In John 15, Jesus speaks of his relationship with his Father and his relationship with us. He is the vine and we are the branches. My favorite part of this and why I think Nephi rejoiced was this promise in verse 7 “If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you.
If we are asking our vine for the nourishment that he promises in his words it will be given to us, causing Nephi and me to express gratitude and feel great delight.