The hard shell represents the worldly person, the fallen man. The shell represents not so much all the illicit and evil lusts and desires but the man and his relationship to them. The outer 'shell' man is attracted to these. This outer man is connected to the 'world' and its evil, just the shell of an egg is connected to the outer world with all its dirt and filth. The inner yolk, is of course protected and remains pure and clean. The yolk is protected from the shell and all that is outside of it by the white of the egg; in our metaphor, by the power of God and His Angels. The inner yolk of a man – unlike an egg – can never be corrupted.
Thus even when we see a most evil man, we can still have compassion. Perhaps even empathy. We can, if we wish, see through the outer shell of the man and recognise that under that shell, no matter how filthy and corrupted, lies another being (hidden) that is worth not just something, but much.
Thus the purpose of life is to release the golden yolk from the bonds of its outer shell. Once released, the true man is realised. This is why the Christian must remain humble – the man that emerges from the egg is no better than any other man (one yolk is no more golden than all the others) and not only that, but the man that emerges has no merit that he can claim for himself – he is, in his purity – a creation of God, and not of one part of him can he claim credit for. Nor can any fault be seen in this created being.
Thus the humility of the reborn Christian (rather than pride). So let us all work with, rather than against God, in the great act of releasing ourselves and others from the hardness and evilness of matter.