Q: "I have found happiness in my life. Why do I need to derive a sense of purpose and happiness from the concept of a 'divine being'?"
You're right, you can find happiness in life without having God in your life. Many people enjoy financial wealth, a loving family, and few problems in life. However, no matter how much happiness you have right now, your life would be more fulfilling with God in it. Knowing God makes a HUGE difference. Think of it like this. Riding a tricycle seems fun to a child, but later if that child grows up and drives a Porsche, the tricycle seems pretty boring. You might think you've got all you'd ever want in life, but know that you might be saying that from the perspective of someone riding a tricycle.
Jesus said that He could fulfill desires in us that nothing else can fulfill.1 That's because we're not just physical beings, we're spiritual beings also. Only God can fulfill our spiritual hunger and thirst completely. And while we may be happy without God, it is only by knowing God that we can have true joy, fulfillment and peace.
And there is also the question of eternity. What if your limited time on earth is a time of testing from God? A test to whether or not you will come to know Him? If so, how are you doing on that exam? Are you (as it says in Romans 1) "suppressing" the knowledge of God? Do you see the evidence for His existence all around you, yet are not wanting to acknowledge Him?
What if you never receive God into your life? Well, as you've mentioned, you can still enjoy the benefits of this life. But know that for you it will end here, because you've chosen to reject God. You've accepted the gift (your existence), but rejected the Giver.
What if God hasn't even shown off yet, but has a lot more creative things planned for the next life? For those who haven't rejected Him? What if that life surpasses this life even more than driving a Porsche surpasses riding a tricycle? If you have rejected God, then you won't be there to see it.
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Footnote: (1) John 4:13-14; 6:35