A man asked Nabi (sallallahu ‘alaihi wasallam), “O Messenger of Allah! What is imaan?” Nabi (sallallahu ‘alaihi wasallam) replied, “When your good deeds please you and your evil deeds grieve you, then you are a believer.” The man asked, “O Messenger of Allah! Then what is sin?” Nabi (sallallahu ‘alaihi wasallam) replied, “When something causes a doubt in your heart, leave it out.” (Musnad Ahmad #22166](http://www.paklinks.com/gs/usertag.php?do=list&action=hash&hash=22166) )
Our imaan (faith) which is hidden within us will allow us to be happy over the good we do, and make us sad over the sin we commit. The expression of joy and sorrow over our good and bad actions respectively are signs of imaan. Let us allow our outward expression of feelings be the signal of the existence of imaan within us. (Expressing sorrow over the sin refers to feeling very guilty and sincerely repenting)
Further, this hadeeth explains the golden principle of “When in doubt, leave it out”. This is a simple criterion for us when we are unsure if something is permissible or not.
@Kinzz
this made clear so many things in my mind 
Alhamdulilah