why lowering your gaze is good for you ?

Lower your gaze...!
Smile and be happy because Allah love the real believers. O you (believing men and women): "lower your gaze from looking at forbidden things.

 Allah says;

O you (Muhammad), Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts). That is purer for them. Verily, Allah is All-Aware of what they do.(24:30)

 And also to you women.

And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, head cover, apron).

And to draw their veils all over Juyûbihinna (i.e their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers, or their husband's fathers, or their sons, or their husband's sons, or their brothers, or their brother's sons, or their sister's sons, or their (Muslim) women (i.e their sisters in Islam), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of feminine sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allah to forgive you all, O believers, that you may be successful. (24:31)

Narrated Abu Sa`id Al-Khudri:

The Prophet (ﷺ) said, 'Beware! Avoid sitting on the roads." They (the people) said, "O Allah s Apostle! We can't help sitting (on the roads) as these are (our places) here we have talks." The Prophet (ﷺ) said, ' l f you refuse but to sit, then pay the road its right ' They said, "What is the right of the road, O Allah's Apostle?" He said, 'Lowering your gaze, refraining from harming others, returning greeting, and enjoining what is good, and forbidding what is evil."
[Sahih al-Bukhari book 79, Hadith 3]

Narrated Abu Sa`id Al-Khudri:

The Prophet (ﷺ) said, "Beware! Avoid sitting on he roads (ways)." The people said, "There is no way out of it as these are our sitting places where we have talks." The Prophet (ﷺ) said, "If you must sit there, then observe the rights of the way." They asked, "What are the rights of the way?" He said, "They are the lowering of your gazes (on seeing what is illegal to look at), refraining from harming people, returning greetings, advocating good and forbidding evil."
[Sahih al-Bukhari book 46, Hadith 26]