Atheist: Objective morality can be arrived at with reason alone and we should all be obligated to live by it. Right is simply what makes the most people happy.
Response: I am not sure you understand the implications of your assertions. Given your worldview you have no reason to believe in a moral imperative that is true for everyone. Who decides? And to whom is everyone accountable? And by what standard? In an accidental universe, whatever happens, just happens. Chemical reactions simply follow the law of physics.
To claim that morality is OBJECTIVE is like saying that they are like the laws of logic, which are true for everyone, regardless of where they were born and what culture they were brought up in. The law of non-contradiction, for example, is true regardless of whether you were born in Mexico or Indonesia. And, as you know, these abstract and immaterial laws of logic do not change with time and are always true without exception. Is that what you mean by objective morality? And what is to prevent your neighbor from creating a different morality than you? Why should your view be the accepted view?
In an accidental universe why would there be universal standards? Therefore it is quite plain that morality could never be objective in a strictly atheistic material universe. To claim something like right is something that makes the most people happy is to make an arbitrary assertion. Why should people be obligated to follow THAT particular preference? And even if they did, they might have a different definition of happiness. And how could one chemical reaction be morally obligated to another? If we are merely the reaction of chemistry over time why does anyone deserve respect?
In an atheistic universe, moral abstractions would simply be the result of a chemical epiphenomena, and thus have zero authority or purpose. We have no obligation to an impersonal moral abstraction that is merely the result of a chemical reaction. How can anyone arrive at OBJECTIVE morals from impersonal matter? Morals are a personal thing and if there are morals that are binding on everyone, they come from God, not from lightning striking swamp gas. Impersonal moral abstractions that are UNIVERSAL have no weight, no meaning apart from the One who gives it.
If Atheism were true, every thought, reaction, emotion, law of logic or moral drive would merely, only and always, be a chemical reaction. And nothing besides. There is no justifiable, scientifically viable answer to the contrary. So I would encourage you to either be consistent with your view and embrace the void by accepting that your life is nothing other than matter and chemical reactions in Sagan's closed universe....or, better yet, by God's grace, when you are awakened from the incoherence of that worldview, may God grant that you follow the evidence to the truth of Jesus Christ, the only Savior.