The unrestrained glance results in the one who looks becoming attracted to what he sees, and in the imprinting of an image of what he sees in his heart. This can result in several kinds of corruption in the heart of the servant. The following are a number a them.
It has been related that the Prophet sallallahu 'alayhi wa sallam once said words to the effect:
"The glance is a poisoned arrow of Shaytan. Whoever lowers his gaze for Allah, He will bestow upon him a refreshing sweetness, which he will find in his heart on the day he meets Him."
Shaytan enters with the glance, for he travels with it, faster then the wind blowing through an empty place. He makes what is seen appear more beautiful then it really is, and transforms it in to an idol for the heart to worship. Then he promises it false rewards, lights the fire of desires within it, and fuels it with the wood of forbidden actions, which the servant would not have committed had it not been for this distorted image.
This distracts the heart and makes it forget its more important concerns. It stands between it and them; and so the heart loses it's straight path and falls into the pit of desire and ignorance. Allah, Mighty and Glorious is He, says:
"And do not obey anyone whose heart We have made forgetful in remembering Us – who follows his own desires, and whose affair has exceeded all bounds." [Al-Qur'an 18:28]
The unrestrained gaze causes all three afflictions. It has been said that between the eye and the heart is an immediate connection; if the eyes are corrupted, then the heart follows. It becomes like a rubbish heap where all the dirt and filth and rottenness collect, and so there is no room for love for Allah, relating all matters to Him, awareness of being in His presence, and feeling joy at His proximity - only the opposite of these things can inhabit such a heart.
Staring and gazing without restraint is disobedience to Allah:
"Tell the believing men to lower their gaze and guard their modesty; that is more purifying for them. Surely Allah is aware of what they do." [Al-Qur'an 24:30]
Only the one who obeys Allah's commands is content in this world, and only the servant who obeys Allah will survive in the next world.
Furthermore, letting the gaze roam free cloaks the heart with darkness. Just as lowering the gaze for Allah clothes it in light. After the above ayah, Allah, the Glorious and Mighty, says in the same Surah of the Qur'an:
"Allah is the light of the heavens and the earth; the likeness of His light is as if there were a niche, and in the niche is a lamp, and in the lamp is a glass, and the glass is as it were a brilliant star, lit from a blessed tree, an olive neither of the east nor of the west, whose oil is well nigh luminous, though fire scarce touched it. Light upon light. Allah guides whomsoever He wants to His light. Allah strikes metaphors for man: and Allah knows all things." [Al-Qur'an 24:35]
When the heart is a light, countless good comes to it from all directions. If it is dark, then clouds of evil and afflictions come from all directions to cover it up.
Letting the gaze run loose also makes the heart blind to distinguishing between truth and falsehood, between the sunnah and innovation: while lowering it for Allah, the Mighty and Exalted, gives it a penetrating, true and distinguishing insight.
A righteous man once said:
"Whoever enriches his outward behaviour by following the sunnah, and makes his inward soul wealthy through contemplation, and averts his gaze away from looking at what is forbidden, and avoids anything of a doubtful nature, and feeds solely on what is halal - his inner sight will never falter."
Rewards for actions come in kind. Whoever lowers his gaze from what Allah has forbidden, Allah will give his inner sight abundant reward.
(s) The Purification of the Soul from the works of Ibn Rajab al-Hanbali, Ibn al-Qayyim al-Jawziyyah and Abu Hamid al-Ghazali