Happiness is a fleeting illusion. When you attain whatever it is you think will make you happy, you often find a disappointing result. Happiness will never make you "happy." What we need is joy.
This promise centers on joy.
In the verses before this promise, Jesus described how He is the vine, and we are the branches. We cannot thrive outside of our union with Christ.
Jesus walked in complete joy because he walked in perfect obedience and union with God.
We rarely think obedience and joy go together. Obedience brings to mind a list of rules and regulations. The dos and don'ts of religion.
Jesus walked in joy because He walked in obedient intimacy with God. Our joy comes as we walk in obedient intimacy with Jesus.
Obedience to Christ is not following a set of rules and regulations. Obedience is walking as Jesus walked. We are called to be followers of Christ, imitators of Him.