"The happiness of God is first and foremost a happiness in His Son. Thus when we share in the happiness of God, we share in the very pleasure that God has in the Son. Ultimately this is what makes the gospel good news. It opens the way for us to see and savor the glory of Christ [which is the Father's glory perfectly reflected]. And when we reach that ultimate goal we will find ourselves savoring the Son with the very happiness that the Father has in the Son" (John Piper, God is the Gospel, pg. 101).
I think I'd tie in Piper's great influence, Jonathan Edwards. In his unpublished essay on the Trinity, he says this about the delight between the Father and Son as it relates to the Holy Spirit: "The Godhead being thus begotten by God's loving an idea of Himself and shewing forth in a distinct subsistence or person in that idea, there proceeds a most pure act, and an infinitely holy and sacred energy arises between the Father and Son in mutually loving and delighting in each other, for their love and joy is mutual, (Prov. 8:30) 'I was daily His delight rejoicing always before Him.' This is the eternal and most perfect and essential act of the Divine nature, wherein the Godhead acts to an infinite degree and in the most perfect manner possible. The Deity becomes all act, the Divine essence itself flows out and is as it were breathed forth in love and joy. So that the Godhead therein stands forth in yet another manner of subsistence, and there proceeds the third Person in the Trinity, the Holy Spirit, viz., the Deity in act, for there is no other act but the act of the will...the Holy Ghost is the deity subsisting in act, or the divine essence flowing out and breathed forth in God's infinite love to and delight in Himself."
While Piper does utilize these Edwards quotes in a footnote (The Pleasures of God, pgs. 44-45), I don't think he makes the application to our lives as he could have. In the one living and true God Who is three Persons, the Holy Spirit is the joy and delight the Father has in the Son (Who is eternally and perfectly reflecting His glory); the Holy Spirit is also the joy and delight the Son has in the Father (Whose glory He eternally and perfectly reflects). The Holy Spirit is the eternal Person of the joy and delight of God. It is not a happiness based on circumstance, but on the unchanging reality of the Father and Son's delight and joy in each other. Nothing in this world or in our lives can change this, for it was the reality before there was space and time. It is eternally unchanging and foundational.
What I wish Piper would have done was make the pastoral application of this truth. This Spirit Who is eternal joy is the same Spirit Who is given to believers. "...the fruit of the Spirit is...joy" (Galatians 5:22). Our reading and consideration of Galatians 5:22 usually leads us to think the Holy Spirit should make us happy - we focus on the emotion opposite depression. This is why I started with the quote I did, though. The Holy Spirit is not a "positive emotion generator" in our lives. He is the joy and delight of the Father for the Son, and the Son for the Father - a delight based on the Son's perfect reflecting of the Father's glory. The Holy Spirit is given to us to seal us in the Father-glorifying Person of the Son. Jesus said of this relationship: "He [the Holy Spirit] will glorify Me [the Son], for He will take what is Mine and declare it to you. All that the Father has is Mine; therefore I said that He will take what is Mine and declare it to you" (John 16:14,15).
The joy that is the fruit of the Holy Spirit is not an unfocused joy (a vague, mystical, inexplicable emotion). The joy that is the fruit of the Holy Spirit is not the result of the material, earthly blessings He might bring into our lives. The joy that is the fruit of the Holy Spirit is the result of Him drawing us into the eternal relationship between the Father and the Son, which is all delight and joy in the glory of the Father in the Person of the Son.
The Spirit uses the means of the Word, prayer, the Lord's Supper, and the Christ-exalting fellowship of the Church to draw us closer to Christ, Who is the object of the Father's eternal delight. In turn, we see the Father in the Son (John 14:9) and experience the eternal joy of the Son in the Father's glory. The 1689 Baptist Confession of Faith describes the Spirit's use of means to bring us grace: "...[the grace of faith] is...increased and strengthened by the work of the Spirit through the ministry of the Word, and also by the administration of baptism and the Lord's Supper, prayer, and other means appointed by God" (14.1). The "grace of faith" is the Spirit's bringing us into the joyous relationship between the Father and the Son. He uses the ordinary (continual, accessible to all believers) means of grace as they are observed in the Church.
The Trinity is eternally joy. In the salvation He has graciously given us, we are brought into His eternally delightful fellowship for His glory and our eternal joy. Do not grieve this Person of joy, but increase your commitment, obedience, and involvement in the Scripture-ordained means He uses to draw you into His joy as He helps you see the Son more and more. "Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. And we all, with unveiled face, beholding the glory of the Lord, are being transformed into the same image from one degree of glory to another. For this comes from the Lord Who is the Spirit" (2 Corinthians 3:17,18).