What comes out of a man, that defiles a man. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. (Mark 7:21-22 NKJV)
Jesus seems to be saying that from deep inside of us, from our sinful hearts, comes the external actions that appear so clearly as sin to others. I hate that. I want to pretend that when I do something bad it was a one-off, an accident, that I was having a bad day or was just surprised and didn't know what to do.