One measure of sincerity is whether or not there is true remorse. Some people say they feel remorse, but what they really feel is just regret at having been caught doing something wrong. They feel bad for themselves because they may have to face someone else's anger, and there may be some kind of unpleasant consequences or a penalty to pay. Maybe they even wish they hadn't done whatever it was they did, but only because they don't like paying the price.
