Fear prevents a person from sin, and today hearts are rusting because of the intensity of sins that we have committed. This is not to say that people are not doing good deeds, because they are. However, good deeds and sins are taking place side by side, which is counterproductive. A good deed is offering prayer in the masjid. However, when walking home our eyes are wandering and looking at forbidden things, and therefore we waste the good effects that being in the masjid had on our hearts.
Likewise, reading the Holy Quran is a good deed but then we also lie throughout the day. Many women pray Tahajjud (the night vigil) but also go out in public with their heads uncovered. Why are people engaged in both good and bad at the same time? This is because we don’t fear Allah as we should.
Fear of Allah is a blessing from the many blessings of Allah and we should request it from Allah and crave it. We ask other things from Allah, and we should also ask to be made fearful of Allah. Allah calls this fear a blessing because it prevents people from sin.
A condemned prisoner does not sit in his cell planning more crime. Crime no longer matters to him since he knows that he is about to die; this causes his heart to be heavy. Likewise, a believer has the Day of Judgment and the Fires of Hell constantly before him and this keeps him from committing sin and therefore protects him.
A young man came to a shaykh and said, “I am a young man with strong desires. I cannot help looking at people when I am in the bazaar. What do I do?” The shaykh handed him a cup of milk that was full to the brim, and told him to deliver it to an address that would take the young man directly through the bazaar. He then instructed the young man that the milk should not spill, and the man said that it would not. The shaykh then summoned one of his students and told him to accompany the man through the bazaar to the destination, and to severely beat him if any milk were to spill.
This man successfully took the milk to the destination and returned to give the good news to the shaykh. The shaykh asked, “Tell me, how many sights and faces did you see in the bazaar?” The man replied incredulously, “Hadrat, I didn’t even realize what was going on around me. I was only fearful of the beating and disgrace that I would have to endure in public if a drop of milk were to spill.” The shaykh smiled and said, “It is the same with a believer. The believer fears that Allah may disgrace him in front of the entire creation on the Day of Judgment if his faith were to spill over from his heart.” Such people save themselves from sin because they constantly remain focused on Allah’s punishment on the Day of Judgment.
(An extract from an article: http://www.tasawwuf.org/writings/sermons/fear_of_allah.htm)