If you let Shaytaan into your heart then what was once beautiful becomes ugly and what was once ugly becomes beautiful. 

“Shaytaan had made their foul deeds fair-seeming to them and diverted them from the Right Way, though they were intelligent people.” [Surah Al Ankabut: Verse 38]

Our job is to sense when shaytaan is knocking on our hearts because he is close, he will whisper in the chest and when someone is knocking on the outside, can they be heard on the inside? Yes! When you open the door so many times to an outsider,  they move in. This is what shaytaan does to our hearts.