Sin promises happiness. You sever the root of sin by a superior promise of a superior joy. Therefore, the pursuit of lasting joy is indispensable in the christian life. Defeat says: Even if the sin I'm planning to do is wrong, the resulting pleasure or security will be worth it. Victory says: I despise your promise, sin! I will pursue true happiness: in God's presence are pleasures for evermore (Ps. 16:11) and He is my exceeding great reward. (Gen. 15:1) As Lewis said, we are far too easily pleased. Al men seek happiness, so Pascal, so becoming or being a christian means not to give up my pursuit of happiness in order to become reformed and more moral. No, it means to finally turn my pursuit on the true satisfying object, to find a deeper and more lasting joy than sin or the world can ever offer or give. Are you a christian and do you desire to conquer sin and live a holy life? Delight yourself in the Lord, and He shall give you the desire of your heart. (Ps 37:4)
If there lurks in most modern minds the notion that to desire our own good and earnestly to hope for the enjoyment of it is a bad thing, I submit that this notion has crept in from Kant and the Stoics and is no part of the Christian faith. Indeed, if we consider the unblushing promises of reward and the staggering nature of the rewards promised in the Gospels, it would seem that our Lord finds our desires, not too strong, but too weak. We are half-hearted creatures, fooling about with drink and sex and ambition when infinite joy is offered us, like an ignorant child who wants to go on making mud pies in a slum because he cannot imagine what is meant by the offer of a holiday at the sea. We are far too easily pleased.
C.S. Lewis - The Weight of Glory, p 26