The actual physical performance is only a part of the totality of any mitzvah.
It is not enough to perform the required action mechanically; one has to do it with a wholeness of spiritual awareness.
Conditioned habit is only one of the impediments on this path.
There are also times when one gets tired, either simple physical tiredness or spiritual weariness.
In addition, many well-intended acts are done under pressure, in haste, or in conditions that distract the heart.
If one tries to maintain a high spiritual poise all the time, one soon discovers that it involves a tremendous effort, and is not always possible.
Furthermore, it may frequently be appropriate for a person, even one of high spiritual sensitivity, to accept the blessedness of routine, the doing for its own sake, in thoughtless simplicity and purity of heart, as an intrinsic part of the fabric of life.
Generally speaking, the tension between deed and intention, between times of inspiration and times of practical action, is also a part of the spiritual way in which a Jewish personality is formed.
–Rabbi Adin Steinsaltz
From "Actions and Intentions," in Teshuvah by Rabbi Adin Steinsaltz