In the Song of the Sea, Moses and the Jewish people sang (Exodus 15:2), "זה א-לי ואנוהו" - "This is my God and I shall glorify Him!" Based on this verse, the Talmud (Shabbos 133b) teaches that we have an obligation to perform mitzvos in a beautiful manner:
זה אלי, ואנוהו - התנאה לפניו במצותֹ עשה לפניו סוכה נאה, ולולב נאה, ושופר נאה, ציצית נאה, ספר תורה נאה וכתוב בו לשמו בדיו נאה, בקולמוס נאה, בלבלר אומן וכורכו בשיראין נאין.
"This is my God, and I will glorify Him." - Be beautiful before Him with the mitzvos. Make before Him a beautiful sukkah, [use] a beautiful lulav, a beautiful shofar, beautiful tzitzis, and a beautiful Torah scroll, and write it for His sake with beautiful ink, with a beautiful reed, and with a skilled scribe, and wrap it with beautiful silks.
This is the concept of hiddur mitzva - the beautification of a mitzva. The Ramchal discusses this concept at some length in Mesillas Yesharim, citing this Talmudic passage (and other relevant passages) and expanding on this idea:
הרי דעת שפתותיהם ז"ל ברור מללו, שאין די בעשות המצוה לבד, אלא שצריך לכבדה ולהדרה. ולהוציא ממי שלהקל על עצמו יאמר אין הכבוד אלא לבני האדם המתפתים בהבלים אלה, אך הקב"ה אינו חושש לזה, כי הוא מרומם מדברים האלה ונשגב מהם, וכיון שהמצוה נעשית לאמתה די בזה, אמנם האמת הוא שהאדון ברוך הוא נקרא אל הכבוד, ואנו חייבים לכבדו, אע"פ שאינו צריך לכבודנו ולא כבודנו חשוב וספון לפניו, ומי שממעט בזה במקום שהיה יכול להרבות, אינו אלא חוטא.
Thus, the opinion of the Sages is very clear, that is not sufficient to simply do the mitzva alone, but it is necessary to honor and glorify it. This is a rejection of the opinion of one who, wishing to be lenient for himself, says that honor is only for human beings, who are seduced by such vanities, but God has no interest in this, for He is exalted far above such matters. Therefore, [he argues,] it is sufficient that the mitzva is simply fulfilled correctly. However, the truth is that God is called (Psalms 29:3) the "God of glory", and we are obligated to honor Him, even though He has no need for our glorification, and our glorification is of no real significance before Him.One who minimizes [his beautification of the mitzvos] when he has the ability to do more is simply a sinner.
The Ramchal then goes on to quote many additional sources that demonstrate the importance of this concept. But, while all of these sources clearly demonstrate that the sinner's argument is indeed false, they never really address the question of why his argument is incorrect. Why isn't it sufficient to simply fulfill the mitzvos precisely as they are commanded? Why is there an obligation to beautify the mitzvos?
Perhaps a hint to the answer can be found in a passage from Maimonides, when he concludes his discussion of the laws of acceptable oils for the meal offerings in the Holy Temple (Hil. Issurei HaMizbei'ach 7:11):
ומאחר שכולן כשרין למנחות למה נמנו, כדי לידע יפה שאין למעלה ממנו והשוה והפחות שהרוצה לזכות עצמו יכוף יצרו הרע וירחיב ידו ויביא קרבנו מן היפה המשובח ביותר שבאותו המין שיביא ממנו, הרי נאמר בתורה והבל הביא גם הוא מבכורות צאנו ומחלביהן וישע י"י אל הבל ואל מנחתו, והוא הדין בכל דבר שהוא לשם האל הטוב שיהיה מן הנאה והטוב, אם בנה בית תפלה יהיה נאה מבית ישיבתו, האכיל רעב יאכיל מן הטוב והמתוק שבשולחנו, כסה ערום יכסה מן היפה שבכסותו, הקדיש דבר יקדיש מן היפה שבנכסיו וכן הוא אומר כל חלב ליי' וגו'.
If they (i.e. kinds of oil) are all valid for the menachos (meal offerings), then why did [the Sages] rank them? So that one can know which is the very best, which are of equal status, and which are inferior. For one who wishes to earn merit should subdue his evil inclination and open his hands wide and bring an offering from the very finest and best of the species that he is bringing. For, behold, it says in the Torah (Genesis 4:4), "And Abel also brought from the from the firstborns of his sheep and from their fats, and God turned to Abel and his offering."
This concept applies to everything that is done for the sake of God, Who is good, that it should be from the finest and the best. When one builds a house of prayer, it should be more beautiful than his house of dwelling. When one feeds the hungry, he should feed them from the best and the tastiest food on his table. When one clothes the naked, cover him with the most beautiful of his garments. When one consecrates an item,he should consecrate the most beautiful of his possessions. And so it says (Leviticus 3:16), "All the fat is God's...."
Maimonides introduces this idea by saying that "one who wishes to earn merit should subdue his evil inclination and open his hands wide." In other words, doing the mitzvos in the most beautiful manner is, fundamentally, about subduing our own nature and changing ourselves for the better. Of course God Himself has no need for glory and honor. In truth, He, in of Himself, has no need for our mitzvos either! The mitzvos were given to us for our benefit. They are what enable us to connect to God.
The same is true for the concept of hiddur mitzva. When we beautify a mitzva, in the same way that we would beautify a gift that we give to someone that we love and respect, we reinforce in our own the minds the reality of God and of our relationship with Him. It is this recognition that is at the core of all Jewish spirituality.
Perhaps this explains why the Talmud finds this concept alluded to in this particular verse, which describes the amazing spiritual revelation that the Jewish people experienced at the splitting of the Red Sea. How can we, today, in any way grasp what it means to declare, "This is my God!" - when "this" refers to a direct spiritual experience? Perhaps the answer is that when we sincerely perform the mitzvos, not as formal legal obligations, but as expressions of a genuine loving relationship, with all the love and honor that we put into our most valued human relationships, then we too can eventually come to experience the personal relationship with God expressed in the statement, "This is my God!"