Parshat Tzitzit has the distinction of being incorporated as part of the daily mitzvah of Keriat Shema. Every night and morning Jews are to recite 3 parshiot: Shema, Vehayah im shamoa, and Vayomer. The reason for this specific order of recitation, even though Vayomer (tzitzit) is written in the Torah before the first paragraph, is given in the first Mishna of the second perek of Berachot: "R' Yehoshua ben Korchah taught that shema precedes vehaya as first one accepts upon himself the yoke of God's sovereignty (kabbalat ol malchut shamayim) and then accepts the yoke of god's commandments. The reason "Vehaya im shamoa" precedes "Vayomer" is that the mitzvah of studying (talmud) Torah, which is found in "Vehaya", applies both by day and by night, whereas the mitzvah of tzitzit applies only by day."
The Rambam in the first chapter (Halachah 2) of the laws of keriat shema teaches, in keeping with the gemara in Berachot 12b, that "shema" is recited first as it contains passages teaching the oneness of God, and the commandment to love Him and study His Torah. Vehaya then contains the charge to observe the mitzvot of the Torah, followed by the parshah of tzitzit as this too discusses this charge. What is found in the third parshah of tzitzit that is not contained in the second parshah of "Vehaya im shamoa"?
The Rav z"l suggested the following explanation. The halachah clearly states that one is obligated to affix tzitzit to a four-cornered garment; one not wearing such a garment is exempt from the Torah obligation. The Torah does not instruct us to wear a four-cornered garment in order to fulfill this mitzvah. (Gemara Menachot 41a warns that at a time when there is Divine anger and punishment, one is punished even for the neglect of an asseh - the failure to observe a positive mitzvah.) It is for this reason that we recite our acceptance of mitzvot twice daily. The first, "Vehaya im shamoa", is for those mitzvot that one must observe, such as shemirat shabbat, kashruth, and returning a lost object. The second acceptance refers to those mitzvot that one does not have to fulfill but wants to. The recitation of Parshat Tzitzit indicates our love for and positive attitude towards mitzvot. Mitzvot are to be viewed as a privilege and not as a burden. Though according to the letter of the law I may be excused from fulfilling a particular mitzvah, I wish to put myself in a situation that will enable me to be obligated to fulfill it. (This applies to mezzuzah and maakah as well.)
Moreover, this follows the powerful lesson recited in Parshat Behaalotcha (8:7) of "Lama nigara?" i.e. Why is it that due to being tameh nefesh, ritually impure after having come into contact with a dead body, we are deprived of the opportunity and privilege of offering korban Pesach? The Jews speaking in the passuk did not have to fulfil the mitsvah of korban Pesach, but they wanted to.
May we be privileged to include these uplifting thoughts in our minds when we recite Parshat Tzitzit daily, recalling our commitment to and acceptance of all mitzvot, not only because we have to, but because we want to!