Why when you seriously practice it starts to seem that it is worse than before? God acts as a fire upon a log of wood on lucky aspirants

Saint John of the Cross

 (from "The dark night of the soul" by John of the Cross):

For the greater clearness of what has been said, and of what has still to be said, it is well to observe at this point that this purgative and loving knowledge or Divine light whereof we here speak acts upon the soul which it is purging and preparing for perfect union with it in the same way as fire acts upon a log of wood in order to transform it into itself; for material fire, acting upon wood, first of all begins to dry it, by driving out its moisture and causing it to shed the water which it contains within itself. Then it begins to make it black, dark and unsightly, and even to give forth a bad odour, and, as it dries it little by little, it brings out and drives away all the dark and unsightly accidents which are contrary to the nature of fire. And, finally, it begins to kindle it externally and give it heat, and at last transforms it into itself and makes it as beautiful as fire. In this respect, the wood has neither passivity nor activity of its own, save for its weight, which is greater, and its substance, which is denser, than that of fire, for it has in itself the properties and activities of fire. Thus it is dry and it dries; it is hot and heats; it is bright and gives brightness; and it is much less heavy than before. All these properties and effects are caused in it by the fire.

In this same way we have to philosophize with respect to this Divine fire of contemplative love, which, before it unites and transforms the soul in itself, first purges it of all its contrary accidents. It drives out its unsightliness, and makes it black and dark, so that it seems worse than before and more unsightly and abominable than it was wont to be. For this Divine purgation is removing all the evil and vicious humours which the soul has never perceived because they have been so deeply rooted and grounded in it; it has never realized, in fact, that it has had so much evil within itself. But now that they are to be driven forth and annihilated, these humours reveal themselves, and become visible to the soul because it is so brightly illumined by this dark light of Divine contemplation (although it is no worse than before, either in itself or in relation to God); and, as it sees in itself that which it saw not before, it is clear to it that not only is it unfit to be seen by God, but deserves His abhorrence, and that He does indeed abhor it