God commanded us to do the mitzvot and Talmud Torah is considered on the highest level. Why is this the case? And why should one learn torah?
The sages provide various reasons for the different mitzvot - I am looking for the reasons for Talmud Torah.
The sages say that תלמוד גדול שהתלמוד מביא לידי מעשה (Kiddushin 40b) this can be understood in the technical way, which makes perfect sense, you cannot fulfil the mitzvoth if you don't know what to do, and you never end, there's always more to it.
But I understand that statment also in the 'emotional' way. let me explain. Once a person asked the Chazon Ish if he should leave his 'seder iyun' to replace it with a rapid learning of halacha or the Daf Yomi, the Chazon Ish answered "if you do so, when will you learn mussar?". (this is found in one of his responsas, I don't remember which one . But you should see his book אמונה וביטחון , part 3; where he expands on this concept)
When you learn torah specially when its in-depht , you identify with it, and that creates passion, consisting in hate of worldly desires, and love for the law, for the mitzvah, for the word of God, and love for God himself*, and that makes you have a very different perspective when doing the mitzvoth.
So תלמוד גדול שהתלמוד מביא לידי מעשה refers also to the Quality of your thoughts and intentions when doing the mitzvah, therefore improving the effect the mitzvah has in you; therefore arising the need in you to learn more, and you get to an endless loop of closeness to God.
*it's not a coincidence that in Kriyat Shma, the mitzvah of learning torah comes right after אהבת ה׳.
בראתי יצר הרע ובראתי לו תורה תבלין
"I have created the yetzer hara (evil inclination), and I created the Torah as its antidote"
The gemara in Berachot 61b recounts a story:
פעם אחת גזרה מלכות הרשעה שלא יעסקו ישראל בתורה בא פפוס בן יהודה ומצאו לרבי עקיבא שהיה מקהיל קהלות ברבים ועוסק בתורה אמר ליה עקיבא אי אתה מתירא מפני מלכות אמר לו אמשול לך משל למה הדבר דומה לשועל שהיה מהלך על גב הנהר וראה דגים שהיו מתקבצים ממקום למקום אמר להם מפני מה אתם בורחים אמרו לו מפני רשתות שמביאין עלינו בני אדם אמר להם רצונכם שתעלו ליבשה ונדור אני ואתם כשם שדרו אבותי עם אבותיכם אמרו לו אתה הוא שאומרים עליך פקח שבחיות לא פקח אתה אלא טפש אתה ומה במקום חיותנו אנו מתיראין במקום מיתתנו על אחת כמה וכמה אף אנחנו עכשיו שאנו יושבים ועוסקים בתורה שכתוב בה כי הוא חייך וארך ימיך כך אם אנו הולכים ומבטלים ממנה על אחת כמה וכמה
Once the Roman government issued a decree forbidding Torah study. Papus ben Yehudah saw Rabbi Akiva conducting Torah classes and asked him, “Do you not fear punishment by law?” Rabbi Akiva answered with a parable: A fox was strolling along the riverbank and noticed fish swimming swiftly from place to place. He asked, “Why are you running?” They replied, “We are afraid of the net that people set up to catch us.” The fox slyly said, “Perhaps it would be wise to ascend to the shore and live together with me as my parents lived with your parents.” The fish responded, “You speak foolishly; if we are afraid in our native habitat, our fear will be even greater on land, where death will be certain.” Similarly, Torah is our source of life and may save us. Without it we will definitely perish. src
i.e. a Jew without torah study is like a fish without water.
In the USA, we find that many communities which were not so strong in torah study could not survive the winds of assimilation and disappeared leaving behind huge synagogues lying empty. While tiny communities which started as a handful of Torah students learning in a kollel have now become huge flourishing Torah communities.
One should perform a mitzva solely because G-d has commanded him to perform it, even if one does not understand why. Thus, Ramba"m Mishnah Torah, Torah study ch. 1 cites sources for the mitzvah. He also explains that one of the main reasons is to be able to teach your children what to do.
Perhaps this sounds "circular", but, it's not. Related to Emilio's answer, one needs to learn what to do before he can do it. Children, mimic what parent's do. So, it's critical that parent's do mitzvoth correctly, so that they will perform them correctly. (How often have we seen adults observe most of Shabbat laws correctly but turn lights on and off because that's what they saw their parents do this?)
This is just one angle. There is another angle that is similar to this, but mentions an "opposite" view that one needs to continuously study so that he doesn't forget what he learns and make a mistake by saying something is permissible when it is forbidden. See Avot D'Rav Natan ch. 24, near the bottom of the page. I wouldn't disregard the beginningof the chapter, either, as this includes many analogies to those who perform mitzvoth but don't have proper Torah knowledge. (This is a common debate in several places in Mishnah, Gemara, and Braitah, etc.) It is interesting to note that almost this entire chapter has sayings from Elisha ben Avuya, who, himself, became a heretic!
