The development of fear of God, or reverence for God, is the purpose of the mitzvot and the ideal to which humanity must strive.

A person acts differently when he stands before the king:

A person does not sit, move, and occupy himself in the same manner when he is alone and at home as he does in the presence of a great king. His speech and cheer when he is among the members of his household and relatives are not the same as when he is in the presence of a king. If he therefore desires to attain human perfection, and to truly be a man of God, he should bear in mind that the great King who is over him, and is always close to him, is greater than any earthly king, even David and Solomon.

The consciousness that God is with us at all times brings on a feeling of trepidation and self-consciousness:

When the perfect ones bear this in mind, they are filled with fear of God, humility, and piety, with true, not [merely] apparent, reverence and respect of God…They understand that by these rules the above-mentioned idea will be firmly established in the hearts of men: That we are always before God, and it is in the presence of His glory that we go to and fro. The great men among our Sages would not uncover their heads because they believed that God’s glory was around them and over them.

The goal of the entire Torah is fear of God:

What I have pointed out to you here is the object of all our religious acts. For by carrying out all the details of the prescribed practices, and repeating them continually, a few pious men may attain human perfection. They will be filled with respect and reverence toward God and, bearing in mind who is with them, they will perform their duty. (Rambam, Guide of the Perplexed III:52)

When a person meditates daily that the Holy One, blessed be He, is found everywhere and observes all human actions, he will be careful not to do anything against God’s will. A person should think deeply about this as he begins his day. Then, an additional small reminder during the rest of the day will help him act properly.

Any Jewish person, whoever he may be, who, for a considerable amount of time every day meditates on how the Holy One, blessed be He, truly fills the upper and lower levels of existence and the actual heaven and earth, and truly “that which fills the entire world is His glory” (Isaiah 6:3), if he observes, sees, and discerns his inner thoughts and feelings, actions and speech, and counts each of his steps, fear of God will be embedded within his heart for the entire day. When he later thinks about this, even for a short period of time at any point during the day, it will cause him to “turn away from evil and do good” (Psalms 34:15) in thought, speech, and deed, not to, God forbid, rebel against the watchful presence of He whose glory fills the entire world. (Rabbi Shneur Zalman of Liadi, Tanya, Likutei Amarim 42)

A person only grasps the deeper level of fear of God, awe, when encountering God’s greatness through intense meditation and great wisdom. However, when one first begins to come close to God he should strengthen his fear of punishment. He must remember that the Holy One, blessed be He, is even particular about details. Therefore, a person needs to be careful not to sin, even unintentionally, and to refrain from eating meat and drinking wine on weekdays, to avoid anger and strictness, and to pursue peace.

One who wants to purify himself and come close to God should first focus on fear of God and attain [the basic level of fear of God,] fear of punishment. For one can only come to the inner level of fear, awe, through great wisdom…Therefore he should be careful not to sin, even unintentionally, and should become totally detached from sin. One must be careful of minor sins, for the Holy One, blessed be He, is extremely particular with the righteous. A person should therefore separate himself from meat and wine on weekdays. He needs to heed the warning: “Turn away from evil and do good; seek peace” (Psalms 34:15). You must seek out peace and become one who pursues it. You must be careful not to be strict in your house about anything small or large, and certainly not to get enraged, God forbid. (Rabbi Yitzchak Luria (Arizal), Etz Ĥayyim, Introduction)

The basic level of fear of God a person must acquire is accepting divine authority, to fear doing something against God’s will. Fear of sin is the necessary basis for keeping all the mitzvot.

The type of fear that each and every person must necessarily have is fear of sin. This is being afraid to do something against God and God’s will. It goes beyond actual sins, malicious or unintentional …. Therefore, this fear, that includes accepting divine authority, “the yoke of the kingdom of heaven,” is necessary for every individual. It is impossible to be without it, for one who lacks fear of sin has thrown off the yoke of divine service and is open to all sorts of evil, God forbid…. This is what is referred to as “fear of heaven,” for it is connected to accepting the yoke of the kingdom of heaven and fear of divine judgment. It is the preliminary stage of divine service and its foundation. As is well known, accepting divine authority is a prerequisite to all mitzvot, as our Sages say (Mekhilta, Masekhta Baĥodesh, Parasha 6): “Accept My kingship, then accept My decrees.” (Rabbi Shalom Dovber Schneersohn, Kuntres HaAvoda 2)

Further reading: For more on God’s observation of all our actions, see p. 173; A Concise Guide to the Sages, p. 367.