Ibn al-Qayyim (may Allaah have mercy on him) said:  

Looking generates thoughts, then thoughts generate ideas, then ideas generate desire, then desires generate will, then the will grows strong and resolves to do the deed, and then the deed will inevitably take place unless there is something to prevent it. Concerning this it was said that patience in lowering the gaze is easier than patience in bearing the pain that comes afterwards. End quote. 

Al-Jawaab al-Kaafi (p. 106).