Most Christians tend to assume, for historical reasons, that matters of natural law can be discussed with atheists. As the historical reasons become less relevant this becomes less and less the case. The problem comes in that in the modern age most atheists are Materialists (that is, they believe that nothing exists besides matter and forces on it, or to put it another way all fields of study are just applied physics). And the problem is that Materialists do not hold that nature is good.
Actually, it is even worse than this, because if they ever think about it, Materialists must hold that nature is evil. That’s not true of Materialism in all possible worlds, but it is true of Materialism in this world. In our world, a Materialist believes that we exist purely as an accident of evolution. Evolution cares about nothing, but in a metaphorical sense it only cares about maximizing our descendants. In no sense does it care whether we’re happy. So all happiness, according to a Materialist, must be something which happened to maximize the number of descendants our ancestors had, and therefore only serves, in us, to (probably) maximize the number of descendants we have. All pleasure is a carrot dangled in front of us to keep us going.
Of course this means that pleasure must be, if not strictly speaking minimized, at least kept relatively small. This is why the Romans would starve and thirst baboons before letting them loose in an arena to kill prisoners for sport. A contented baboon usually won’t see the point in ripping a man limb from limb. Contentment is the enemy of effort, and in the great battle of all against all that is nature, quite a bit of effort is needed. Evolution must ensure that happiness won’t last.
The Materialist is, therefore, in the position of needing to cheat his creator in order to be as happy as possible. Nature is not the source of man’s happiness, it is a limit to be overcome and an enemy to be fought.
Of course it is not really possible to beat one’s creator forever. In the end, the creature will always lose. This is why the right to suicide is so often of great important to the Materialist. When you can no longer cheat life, the only thing left to do is to cheat death.