"And these words [shall be]" — the words as they are written, that one should not recite the recitation of the Shema in reverse order. "which I command you today" — let them always be new to you, as though today you were commanded concerning them. "upon your heart" — that you must direct your heart to its recitation.
Reading the Shema in Order and Keeping Torah Forever New
Midrash Aggadah, Deuteronomy 6:6
והיו הדברים הלאה. דברים ככתבם שלא יקרא קרית שמע למפרע: אשר אנכי מצוך היום. לעולם יהיו חדשים אצלך כאילו היום נצטוית עליהם: על לבבך. שצריך לכוין לבך לקרייתה: