1. We can’t escape longings for happiness: “The longing to be happy is a universal human experience, and it is good, not sinful.”
2. This longing is God-given: “We should never try to deny or resist our longing to be happy, as though it were a bad impulse. Instead we should seek to intensify this longing and nourish it with whatever will provide the deepest and most enduring satisfaction.”
3. Only God can ultimately best satisfy this longing: “The deepest and most enduring happiness is found only in God. Not from God, but in God.”
4. Joy in God is cultivated in many ways: “The happiness we find in God reaches its consummation when it is shared with others in the manifold ways of love.”
5. God is most glorified in us, when we are most satisfied in Him: “To the extent we try to abandon the pursuit of our own pleasure, we fail to honor God and love people. Or, to put it positively: the pursuit of pleasure is a necessary part of all worship and virtue. That is, ‘The chief end of man is to glorify God BY enjoying him forever.’”
Note: I would rather say that God is most glorified in us when we are most in love with Him, but since we will only love Him to the degree that we enjoy Him and are satisfied by Him, it is a mere matter of emphasis. Piper's teaching is not in conflict with this idea, rather, his expression of it is more nuanced. It is true that we love God no further than we delight in Him, and thus Piper's popular maxim, "God is most glorified in us when we are most satisfied in Him" is certainly true. In fact, I don't know of a better way to describe what it is like to love someone better than to say, "My soul most deeply delight in her with vigerous satisfaction."