To know Him, to love Him, to serve Him in this world, and to be happy with Him forever in the next.
If we know God, it is impossible not to love Him. If we love God, it is impossible not to serve Him. And if we serve God, we will infallibly enjoy eternal happiness with Him.
The key to all happiness, then, is a knowledge of God.
If you would be happy, if you would solve all of your problems, if you would answer all your questions, learn to know God.