As-Salaam alaykum,
A man once asked the Prophet, Sallallahu alaihi Wasallam, "O Messenger of Allah! What is Imaan [faith]"? He replied: "When your good deeds please you and your evil deeds grieve you, then you are a believer."

The man further asked: "O Messenger of Allah, then what is sin?" The Prophet, alaihis-Salaat Was-Salaam, replied: "When something causes a doubt in your heart, leave it out." [Musnad Ahmad, No. 22166].

Thus expression of joy and sorrow over our good and bad actions respectively are signs of faith (Imaan)... but expressing sorrow over sin, specifically, refers to a feeling of guilt and should prompt sincere repentance.