If there is no God, why by good?

Richard Dawkins responds to the quesion that, "if there is no God, why by good?"

Posed like that, the question sounds positively ignoble. When a religious person puts it to me in this way (and many of them do), my immediate temptation is to issue the following challenge: 'Do you really mean to tell me the only reason you try to be good is to gain God's approval and reward, or to avoid his disapproval and punishment? That's not morality, that's just sucking up, apple-polishing, looking over your shoulder at the great surveillance camera in the sky, or the still small wiretap inside your head, monitoring your every move, even your every base thought.' As Einstein said, 'If people are good only because they fear punishment, and hope for reward, then we are a sorry lot indeed.'

(p.259, The God Delusion)

It is a fair point, but Lewis explains why Christians motivations to be moral are not necessarily like Dawkins describes:

We must not be troubled by unbelievers when they say that this promise of reward makes Christian life a mercenary affair. There are different kinds of rewards. There is the reward which has no natural connection with the things you do to earn it and is quite foreign to the desires that ought to accompany those things. Money is not the natural reward of love; that is why we call a man mercenary if he marries a woman for the sake of her money. But marriage is the proper reward for a real lover, and he is not mercenary for desiring it. [...] The proper rewards are not simply tacked on to the activity for which they are given, but are the activity itself in consummation.

(The Weight of Glory)

I think there will be varying degrees of reward in the New Creation, but it will be the reward of the joy in the fruit of the works we have done. E.g. if I encourage a brother or sister, or I see someone come to faith in some small way due to God's work through me then I will rejoice in the New Creation in their joy and happiness. The more such works, the more causes of joy I have.

After all. Jesus laid down his life for the reward of "the joy set before him" (Hebrews 12:2), and that joy was that he would have a bride. Paul laboured to plant churches for the reward of his joy in the joy of the churches who would know Christ as a result (1 Thessalonians 2:19).

There is nothing mercenary about that.