Because if God exists, and created everything—including you—then God has a claim on your life, a plan for it, and has something to say about how you live it. God also has something to say about life and death, social order, morality, responsibility, the afterlife, and our eternal destiny.
The bottom line is that God is the source-of life, of happiness, of fulfillment, of purpose, of love… Without Him your life will always feel incomplete and empty. God also holds the keys to the future, both short and long, and He wants to have a relationship with you that lasts forever.