There are many reasons for this. One could be that the spiritual life has not been nourished properly by prayer and sacrifice. So, there's a fundamental disconnect from the source of spiritual life.
A person in this predicament then develops complacency and lukewarmness, a kind of spiritual anemia, which can still deteriorate into the worse state of unbelief that can attract all sorts of anomalies. The spiritual growth is stunted and arrested.
It is because of this possible scenario that we are encouraged to pray without ceasing and to be generous in our sacrifices and mortification and penance. Christ himself told to us clearly: "If anyone has to follow me, he should deny himself, take up his cross, and follow me."
Another cause for the weakening of the spiritual life could be that even while some form of prayer and sacrifice is done, there is no recourse to the sacraments nor the study of doctrine and its application to life. And so there's a discrepancy between faith and life, intentions and deed, words and action.
A person in this situation often resorts to deception and plays the game of hypocrisy, or at best, he is a dreamer who fails to take up the means to realize his dream. A double life often ensues.
A person in this situation often plays up his natural talents-his intelligence, for example, or his good looks, or his histrionic powers plus the clever use of gimmicks and other devices-to cover up the emptiness of his spiritual life. He can attract many people, except God and those who can read what is really inside his heart.
His purely intellectual understanding of things, while considered deep and vast, if not complicated, according to worldly standards, would be cold and sweeping, without the refinement produced by the charity and mercy of God.
He is prone to lecturing and scolding people. He can drip in self-righteousness. Unfortunately, many people have come to me to complain about certain priests who tend to be scolding them during homilies, or who deliver their homilies with clear absence of a soul. This has to be looked into. This is a serious problem.
If not that, then another possibility is that his words would be so crafted to make them attractive in a pompous way. As Shakespeare would put it: "It is a tale, told by an idiot, full of sound and fury, signifying nothing." He can manage to charm the naïve and the highly impressionable and excitable people.
In a sense, he is a more dangerous person, since he can mislead a lot of people. He fits the image of the hireling mentioned in the gospel, the antithesis of the good shepherd who would go to extremes to save a lost sheep.
Another cause of weakening of the spiritual life would be worldly attachments, and the cares and concerns of our earthly life that can dull our natural longing for God. The lust for money, for power and fame, are the usual culprits.
We need to be most careful when signs of these dangers start to appear. We have to nip them in the bud with God's grace and our all-out effort. That effort, which can go to an extraordinary degree, would be all worthwhile, since the spiritual life is the most fundamental aspect of our life, it being our very link and necessary contact with God, our Father and Creator.
Insofar as God is concerned, we have to convince ourselves that he has given us everything to assure us of our salvation, and therefore to help us in our spiritual life. All we have to do is to rev up our faith and to do the implications and consequences of that faith.
But there is still one aspect of the weakening of the spiritual life that is most intriguing. It is the case of a person who seems to have and to be doing already everything that we could consider to be healthy spiritual life.
He prays, he offers sacrifices, he goes to the sacraments, he is knowledgeable about the doctrine of the faith, he works well and has many virtues, he does some apostolate, etc. But he knows there is still something missing in him that he has to learn to give, always with God's grace.
It is because of this that he also knows that while he is already doing a lot, he can still be afflicted with a persistent misery that he seems unable to get rid of. Worse, that he is not yet giving his all. This is the greatest challenge in the spiritual life.