We can think of it this way. Lewis talks about when he was a child and would get a toothache. He says he could go to his mother and ask her for medicine to help with the pain, but he would never do this until the pain became unbearable. He would suffer through it until he could stand it no longer because he knew that once he told his mother about the toothache, she would give him medicine, but she would also do something else: take him to the dentist to fix the root of the problem. Then the dentist would inevitably poke around at all the rest of his teeth until they had set all of them right. He could not get what he wanted (relief from the pain) without getting something more that he did not want.
This is sort of how God is. We go to him to be cured of a particular sin that shames us or that makes daily life difficult. "Well, He will cure it all right, but he will not stop there." This is why Jesus tells us to count the cost before we choose the path of Christianity.
And it's important for us to remember that perfection is what we Christians are in for because otherwise we find ourselves getting to a certain point and then pulling back or resisting. We overcome a few sins and become a little better and then start to feel we are now good enough. We have no intentions of becoming saints; we only want to be decent people.
But it is not about what we want for ourselves. It is what the Creator of our Selves intended for us when He made us. "We may be content to remain what we call 'ordinary people,' but He is determined to carry out a quite different plan. To shrink back from that plan is not humility; it is laziness and cowardice. To submit to it is not conceit or megalomania; it is obedience." Sometimes the process is rough and painful and difficult, but we must see that it is only because God is forcing us up, onward, to a better and higher level; putting us "into situations where [we] will have to be very much braver, or more patient, or more loving, than [we] ever dreamed of being before. It seems to us all unnecessary: but that is because we have not yet had the slightest notion of the tremendous thing He means to make of us."