The Rambam writes (Hilchos Tefillin 4:25): “The holiness associated with tefillin is very great. As long as a person is wearing tefillin on his head and arm, he will be humble and G-d-fearing and will not be drawn to frivolous behavior or empty speech. He will not turn his thoughts to evil matters, but rather will direct his heart to words of truth and justice. Accordingly, a person should try to wear [tefillin] throughout the entire day, for this is the mitzvah associated with them. Among the praises conveyed upon Rav, the student of Rabbeinu Hakadosh, was that he was never seen walking four amos [cubits] without [reciting words of] Torah, without tzitzis, and without tefillin.”
Nowadays, it is accepted to put tefillin on for Shacharis and remove them after davening is over. Why was this mitzvah limited to a short hour (or even less!) in the morning?
The Shulchan Aruch (Orach Chaim 37:2) explains that indeed, the basic halachah requires donning tefillin all day. However, since they require continuous sanctity of thought and cleanliness of the body, and nowadays people aren’t focused on carrying Hashem’s name all day, we only wear tefillin during davening or reciting krias shema, which is when we are fully concentrated on Hashem.
The Chida (Birkei Yosef 37:3) and Mishnah Berurah (37:7) write that besides davening with tefillin, anshei ma’aseh [worthy people] would also learn a little Torah while still wearing his tefillin. The Chida (Machzik Berachah 25:17) and Ben Ish Chai (Year 1, Chayei Sarah 11) explain that it is impossible to attain spiritual perfection without learning Torah with tefillin.
The Smag (Aseh 3) was asked what one should do if he find himself unable to remain focused on his tefillin even during davening. He answers that even the most wicked person can retain awareness of tefillin just during davening. He notes as proof that no one turns down the honor to carry the sefer Torah out of concern he may forget what he is holding. If this is true for a large sefer Torah with many passages, it is all the more so true for tefillin which contain only four Torah sections. Everyone can concentrate on tefillin for a short time.
