Genesis 4:7, 7 If you do well, will your face not be cheerful? And if you do not do well, sin is lurking at the door; and its desire is for you, but you must master it.” - NASB
This verse speaks to me. There is a peace and calm within me when serving the Lord and can't help showing in thy face. There is a tension within me when I let down my guard, especially in public, and it will show in thy face. This happened at the airport one morning and while getting frustrated, another passenger asked, "what's wrong?"