Pesikta Zutrasa (Lekach Tov), Shelach 112b

We learned over there in Maseches Berachos:

R’ Yehoshua ben Korchah said: Why does Krias Shema come before “V’haya Im Shamoa”? So that a person first accepts upon himself the yoke of Malchus Shamayim, and only after that, the yoke of mitzvos.

And why does “V’haya Im Shamoa” come before “Vayomer”? Because “V’haya Im Shamoa” applies both by day and by night, whereas “Vayomer” only applies during the day, since the mitzvah of tzitzis only applies by day and not at night.

Another pshat: “U’reisem oso u’zechartem es kol mitzvos Hashem”—this teaches that someone who is mekayem the mitzvah of tzitzis, it’s as if he was mekayem the whole Torah.

And the number of tzitzis is also a remez to the taryag mitzvos: The gematria of “tzitzis” is 600, plus the 8 strings and 5 knots, which adds up to 613. Similarly, “Torah” with “Anochi” and “Lo Yihiyeh Lecha” equals taryag, and “Bris” with milah also equals taryag.

“And lo sasuru acharei levavchem v’acharei eineichem”—what does “acharei levavchem” mean? This refers to kefirah, like the pasuk says in Koheles: “U’motzei ani mar mimaves es ha’isha”—this refers to kefirah, which traps a person like a woman who is a snare. And that’s why Shlomo Hamelech said in Mishlei: “Stay far away from her, don’t even go near the entrance of her house.”

“And acharei eineichem”—this refers to znus, like we see by Shimshon, who said, “Take her for me, because she is yashar in my eyes.”

“And asher atem zonim achareihem”—this refers to avodah zarah, like it says, “And they were mezaneh after the Ba’alim.”

R’ Nosson says: A person shouldn’t drink from his own cup while thinking about his friend’s cup.

Another pshat in “Lo sasuru acharei levavchem v’acharei eineichem”—this teaches that a person’s eyes follow his heart.

R’ Yishmael says: “Lo sasuru acharei levavchem”—like it says in Koheles, “Rejoice, young man, in your youth,” which is why it says “acharei levavchem.”

Another pshat: “Acharei levavchem”—this teaches that a person has two hearts, a yetzer tov and a yetzer hara. But l’asid lavo, a person will only have a yetzer tov, like it says in Yeshaya, “U’re’isem u’sas libchem”—anytime it’s talking about Hashem, it only mentions one heart, but by people, there are two hearts.