Rav Wolbe (Alei Shur vol 2):On the narrow path to Truth in serving G‑d there is a major impediment which is called “frumkeit” (religiosity) – a term which has no clear and exact translation. “Frumkeit is the natural urge and instinct to become attached to the Creator. This instinct is also found amongst animals. Dovid said, “The lion cubs roar for their prey and ask G‑d for their food” (Tehilim 104:21). “He gives to the beast his food and to the young ravens who call to Him” (Tehilim 247:9). There is no necessity why these verses should be understood as metaphors [and therefore they will be read according to their literal meaning]. Animals have an instinctive feeling that there is someone who is concerned that they have food and this is the same instinct that works in man – but obviously at a higher level. This natural frumkeit helps us in serving G‑d. Without this natural assistance, serving G‑d would be much more difficult.
However this frumkeit, as in all instinctive urges that occur in man, is inherently egoistic and self-centered. Therefore frumkeit pushes man to do only that which is good for himself. Activities between people and actions which are done without ulterior motivations are not derived from frumkeit. One who bases his service of G-d entirely on frumkeit remains self-centered. Even if a person places many pious restrictions on himself – he will never become a kind person and he will never reach the level of being pure motivated. This is why it is necessary that we base our service of G-d on commonsense (da’as). (Study Sotah 22b lists 7 types of activities which it labels as foolish piety. Each one of them is a manifestation of frumkeit without commonsense). Commonsense has to direct our service of G-d. From the moment we desert commonsense and act only according to frumkeit, our Divine service becomes corrupted. This is true even for a person on the level of a Torah scholar. [...]