The argument from beauty has a certain intuitive strength. It is roughly as follows: Beauty evokes longing. When we see or hear something of great beauty we long for something beyond what we think is beautiful. A great piece of music impels us to desire for an experience beyond the one we are having. Of course, we can just say it is synapses and endorphins, but when we do we neuter the power of the experience. Perhaps, Lewis suggests, the beauty we are glimpsing is not in the music or painting or mountain-top view, but from somewhere else mediated through our experience. Lewis concluded that there is no way to satisfy the longing evoked by beauty apart from a transcendent reality. That reality, for Lewis, is the Christian reality, God's reality.
Either the longing we have is misplaced (there is nothing beyond the ink on the page, the grass on the hillside, or the sound waves interacting with our ear drums) or there is a transcendent reality. If there is no transcendent reality, then there is no way to satisfy the longing. It is better to have a longing that is able to be satisfied than a longing that can never be satisfied. Having a longing that cannot be satisfied is what we mean by hopeless. Furthermore, it would be better to have unsatisfiable longing without having any inkling of anything that could, if it existed, satisfy it than to have a longing that could be satisfied if some reality did exist but one is fairly certain it does not exist. The latter is more than hopeless. It is helpless hopelessness.
Perhaps, one might say that there is no need to long for anything beyond the immediate experience in order to enjoy the experience. One can be satisfied with the fact that certain perceptual inputs produce certain pleasant sensations in the body. But that is to say that not only is there nothing transcendent that could, if it existed, satisfy the longing but also that the experience itself is reducible to physical causes and effects and has no quality other than the movement of material entities. If so then there is, in principle, nothing to differentiate one experience from another, no qualitative difference between catching a cold and falling in love.