If there is no God, it is equally reasonable to commit acts of good as it is to commit acts of evil. If there is no universal law of morality, then any action is permissible and moral relativism takes its place. There would be no reason to feel guilty about lying, cheating, stealing, etc. Why? Because my law operates for me, and I can do it as I see fit. The reason why God exists because there is love. Love is beyond matter and energy. You are not your biological components, neurons, etc. They are aspects of you that represent your physical being, but that is all. The love for someone who hurts you is not advantageous for your survival. Loving someone who is dying on their death bed despite your efforts to prolong their life (expensive medical bills, countless doctor check-ups, etc.) is not in your best interest, but you'd feel an immense guilt if you didn't do everything you could to help them. Why? Because love transcends the physical. It's spiritual in nature. Every now and then, people experience things "out of this world". It's out of the physical. Why is that so?