Why is it, you ask, that one can pray for so many years with a prayer book, and still not have prayer in his heart? I think the reason is that people only spend a little time lifting themselves up to God when they complete their prayer rule, and in other times, they do not remember God. For example, they finish their morning prayers, and think that their relation to God is fulfilled by them; then the whole day passes in work, and such a person does not attend to God. Then in the evening, the thought returns to him that he must quickly stand at prayer and complete his evening rule. In this case, it happens that even if the Lord grants a person spiritual feelings at the time of the morning prayer, the bustle and business of the day drowns them out. As a result, it happens that one does not often feel like praying, and cannot get control of himself even to soften his heart a little bit. In such an atmosphere, prayer develops and ripens poorly. This problem (is it not ubiquitous?) needs to be corrected, that is, one must ensure that the soul does not only make petition to God when standing in prayer, but during the whole day, as much as possible, one must unceasingly ascend to Him and remain with Him.
—St. Theophan the Recluse, On prayer, Homily 2